MySQL语句使用。，

文章由LinuxBoy分享于2019-11-24 03:11:35热评（282）

MySQL语句使用。，

MySQL的DDL、DML、DQL语句和单表增、删、改、查
- 实验准备：
- 实验开始：
  - DDL语句
  - DML语句
  - DQL语句
  - 单表操作的分组统计

MySQL的DDL、DML、DQL语句和单表增、删、改、查

实验准备：

实验开始：

DDL语句

表：二维关系 设计表：遵循规范
定义：字段，索引
字段：字段名，字段数据类型，修饰符
约束，索引：应该创建在经常用作查询条件的字段上

MariaDB [(none)]> create database testdb1;     (分号为结束符)
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show databases;  （列出所有数据库）
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| testdb1            |
+--------------------+
4 rows in set (0.00 sec)

[root@centos7 ~]#tree /data/mysql/testdb1/   （其实就是在MySQL下面建立了一个目录）
/data/mysql/testdb1/
└── db.opt

[root@centos7 ~]#cat /data/mysql/testdb1/db.opt （也可以查看里面的内容，字符集也可以看到）
default-character-set=latin1
default-collation=latin1_swedish_ci
0 directories, 1 file

MariaDB [testdb1]> drop database testdb1;   （删除数据库，同时MySQL下面的testdb1目录也会删除）
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| hellodb            |
| information_schema |
| mysql              |
| performance_schema |
+--------------------+
4 rows in set (0.00 sec)

MariaDB [(none)]> show create database testdb1;   (查看testdb1的数据库字符集)
+----------+--------------------------------------------------------------------+
| Database | Create Database                                                    |
+----------+--------------------------------------------------------------------+
| testdb1  | CREATE DATABASE `testdb1` /*!40100 DEFAULT CHARACTER SET latin1 */ | （这里最后的为字符集既latin1这个字符集不支持汉字的。）
+----------+--------------------------------------------------------------------+
1 row in set (0.00 sec)

(这样也可以查看比较好辨认)
MariaDB [(none)]> show create database testdb1\G;
*************************** 1. row ***************************
       Database: testdb1
Create Database: CREATE DATABASE `testdb1` /*!40100 DEFAULT CHARACTER SET latin1 */
1 row in set (0.00 sec)

MariaDB [(none)]> show create database mysql; （查看MySQL的数据库字符集）
+----------+------------------------------------------------------------------+
| Database | Create Database                                                  |
+----------+------------------------------------------------------------------+
| mysql    | CREATE DATABASE `mysql` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [(none)]> alter database testdb1 CHARACTER SET utf8mb4; (这个字符集支持全球文字还支持表情包)
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show create database testdb1;
+----------+---------------------------------------------------------------------+
| Database | Create Database                                                     |
+----------+---------------------------------------------------------------------+
| testdb1  | CREATE DATABASE `testdb1` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+---------------------------------------------------------------------+
1 row in set (0.00 sec)

[root@centos7 ~]#cat /data/mysql/testdb1/db.opt   (文件也会修改)
default-character-set=utf8mb4
default-collation=utf8mb4_general_ci

MariaDB [(none)]> use testdb1;
Database changed
MariaDB [testdb1]> create table test ( id int unsigned auto_increment primary key,name varchar(10) not null,mobile char(11) not null );
Query OK, 0 rows affected (0.01 sec)

MariaDB [testdb1]> desc test;   （查看表结构）
+--------+------------------+------+-----+---------+----------------+
| Field  | Type             | Null | Key | Default | Extra          |
+--------+------------------+------+-----+---------+----------------+
| id     | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| name   | varchar(10)      | NO   |     | NULL    |                |
| mobile | char(11)         | NO   |     | NULL    |                |
+--------+------------------+------+-----+---------+----------------+   
3 rows in set (0.00 sec)



MariaDB [hellodb]> show tables;   （列出当前库里的所有表）
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| scores            |
| students          |
| teachers          |
| toc               |
+-------------------+
7 rows in set (0.01 sec)

(查看表内容，先要进入存在这个表的数据库)  这个表是我从外部导入进来的没有手工创建，需要的话下次我贴出来。
MariaDB [(none)]> use hellodb
Database changed
MariaDB [hellodb]> select * from students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name          | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   | 100 | M      |    NULL |      NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)

（创建表获取帮助命令）
MariaDB [hellodb]> help create table;
Name: 'CREATE TABLE'
Description:
Syntax:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    (create_definition,...)
    [table_options]
    [partition_options]

Or:

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(create_definition,...)]
    [table_options]
    [partition_options]
    select_statement

Or:

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    { LIKE old_tbl_name | (LIKE old_tbl_name) }

MariaDB [hellodb]> create table newstudents select * from students;  （通过查询现存表创建；新表会被直接插入查询而来的数据）
Query OK, 25 rows affected (0.01 sec)
Records: 25  Duplicates: 0  Warnings: 0

MariaDB [hellodb]> show tables;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| newstudents       |
| scores            |
| students          |
| teachers          |
| toc               |
+-------------------+
8 rows in set (0.00 sec)

MariaDB [hellodb]> select * from newstudents;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name          | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   | 100 | M      |    NULL |      NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)

（注意：这样不会把主键之类的复制过来，只是复制数据，表结构不一样）如下：
MariaDB [hellodb]> desc newstudents;
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| StuID     | int(10) unsigned    | NO   |     | 0       |       |
| Name      | varchar(50)         | NO   |     | NULL    |       |
| Age       | tinyint(3) unsigned | NO   |     | NULL    |       |
| Gender    | enum('F','M')       | NO   |     | NULL    |       |
| ClassID   | tinyint(3) unsigned | YES  |     | NULL    |       |
| TeacherID | int(10) unsigned    | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
6 rows in set (0.00 sec)

MariaDB [hellodb]> desc students;
+-----------+---------------------+------+-----+---------+----------------+
| Field     | Type                | Null | Key | Default | Extra          |
+-----------+---------------------+------+-----+---------+----------------+
| StuID     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| Name      | varchar(50)         | NO   |     | NULL    |                |
| Age       | tinyint(3) unsigned | NO   |     | NULL    |                |
| Gender    | enum('F','M')       | NO   |     | NULL    |                |
| ClassID   | tinyint(3) unsigned | YES  |     | NULL    |                |
| TeacherID | int(10) unsigned    | YES  |     | NULL    |                |
+-----------+---------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)

通过复制现存的表的表结构创建，但不复制数据  
MariaDB [hellodb]> create table newstudents2 like students;
Query OK, 0 rows affected (0.01 sec)

MariaDB [hellodb]> select * from newstudents2;   （这里查看数据是没有的）
Empty set (0.00 sec)

MariaDB [hellodb]> desc newstudents2;    （但是有表结构）
+-----------+---------------------+------+-----+---------+----------------+
| Field     | Type                | Null | Key | Default | Extra          |
+-----------+---------------------+------+-----+---------+----------------+
| StuID     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| Name      | varchar(50)         | NO   |     | NULL    |                |
| Age       | tinyint(3) unsigned | NO   |     | NULL    |                |
| Gender    | enum('F','M')       | NO   |     | NULL    |                |
| ClassID   | tinyint(3) unsigned | YES  |     | NULL    |                |
| TeacherID | int(10) unsigned    | YES  |     | NULL    |                |
+-----------+---------------------+------+-----+---------+----------------+
6 rows in set (0.00 sec)

查看支持的engine类型：
MariaDB [hellodb]> show engines;

+--------------------+---------+----------------------------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                                          | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------------------------+--------------+------+------------+
| CSV                | YES     | Stores tables as CSV files                                                       | NO           | NO   | NO         |
| MRG_MyISAM         | YES     | Collection of identical MyISAM tables                                            | NO           | NO   | NO         |
| MyISAM             | YES     | Non-transactional engine with good performance and small data footprint          | NO           | NO   | NO         |
| SEQUENCE           | YES     | Generated tables filled with sequential values                                   | YES          | NO   | YES        |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                                               | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables                        | NO           | NO   | NO         |
| Aria               | YES     | Crash-safe tables with MyISAM heritage                                           | NO           | NO   | NO         |
| InnoDB             | DEFAULT | Supports transactions, row-level locking, foreign keys and encryption for tables | YES          | YES  | YES        |
+--------------------+---------+----------------------------------------------------------------------------------+--------------+------+------------+
8 rows in set (0.00 sec)

查看表：
MariaDB [hellodb]> show tables ;   （列出当前数据库中的所有表）
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| newstudents       |
| newstudents2      |
| scores            |
| students          |
| teachers          |
| toc               |
+-------------------+
9 rows in set (0.00 sec)

MariaDB [hellodb]> show tables from mysql;   （指明数据库查看这个数据库的表）
+---------------------------+
| Tables_in_mysql           |
+---------------------------+
| column_stats              |
| columns_priv              |
| db                        |
| event                     |
| func                      |
| general_log               |
| gtid_slave_pos            |
| help_category             |
| help_keyword              |
| help_relation             |
| help_topic                |
| host                      |
| index_stats               |
| innodb_index_stats        |
| innodb_table_stats        |
| plugin                    |
| proc                      |
| procs_priv                |
| proxies_priv              |
| roles_mapping             |
| servers                   |
| slow_log                  |
| table_stats               |
| tables_priv               |
| time_zone                 |
| time_zone_leap_second     |
| time_zone_name            |
| time_zone_transition      |
| time_zone_transition_type |
| user                      |
+---------------------------+
30 rows in set (0.00 sec)

查看表结构：

MariaDB [hellodb]> desc students;
+-----------+---------------------+------+-----+---------+----------------+
| Field     | Type                | Null | Key | Default | Extra          |
+-----------+---------------------+------+-----+---------+----------------+
| StuID     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| Name      | varchar(50)         | NO   |     | NULL    |                |
| Age       | tinyint(3) unsigned | NO   |     | NULL    |                |
| Gender    | enum('F','M')       | NO   |     | NULL    |                |
| ClassID   | tinyint(3) unsigned | YES  |     | NULL    |                |
| TeacherID | int(10) unsigned    | YES  |     | NULL    |                |
+-----------+---------------------+------+-----+---------+----------------+

表结构字段的意思。
1. Field:字段表示的是别名
2. Type:字段表示的是列的数据类型
3. Null :字段表示这个列是否能取空值
4. Key :在mysql中key 和index 是一样的意思，这个Key列可能会看到有如下的值：PRI(主键)、MUL(普通的b-tree索引)、UNI(唯一索引)
5. Default: 列的默认值
6. Extra :其它信息

查看指定表的创建命令：  （既创建过程）
MariaDB [hellodb]> show create table students;
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                                                                                                                                                                                                    |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| students | CREATE TABLE `students` (
  `StuID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `Name` varchar(50) NOT NULL,
  `Age` tinyint(3) unsigned NOT NULL,
  `Gender` enum('F','M') NOT NULL,
  `ClassID` tinyint(3) unsigned DEFAULT NULL,
  `TeacherID` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`StuID`)
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=utf8 |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

查看看表的状态：（查看指定某个表状态要加单引号）
MariaDB [hellodb]> SHOW TABLE STATUS LIKE 'students'\G; 
*************************** 1. row ***************************
           Name: students
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 25
 Avg_row_length: 655
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: 26
    Create_time: 2019-11-20 21:19:51
    Update_time: NULL
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)

查看库中所有表的状态：
MariaDB [hellodb]> show table status from hellodb\G;
*************************** 1. row ***************************
           Name: classes
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 8
 Avg_row_length: 2048
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: 9
    Create_time: 2019-11-20 21:19:51
    Update_time: NULL
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 
*************************** 2. row ***************************
           Name: coc
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 14
 Avg_row_length: 1170
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: 15
    Create_time: 2019-11-20 21:19:51
    Update_time: NULL
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 
.
.
.(省略证明我不是来混行数的)
.
.
*************************** 8. row ***************************
           Name: teachers
         Engine: InnoDB   （这里是使用什么存储引擎的意思）
        Version: 10
     Row_format: Dynamic
           Rows: 4
 Avg_row_length: 4096
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: 5
    Create_time: 2019-11-20 21:19:51
    Update_time: NULL
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 
*************************** 9. row ***************************
           Name: toc
         Engine: InnoDB
        Version: 10
     Row_format: Dynamic
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: 1
    Create_time: 2019-11-20 21:19:51
    Update_time: NULL
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 
9 rows in set (0.00 sec)

删除表：
MariaDB [hellodb]> show tables;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| newstudents       |
| newstudents2      |
| scores            |
| students          |
| students2         |
| teachers          |
| toc               |
+-------------------+
10 rows in set (0.00 sec)

MariaDB [hellodb]> drop table newstudents2;
Query OK, 0 rows affected (0.00 sec)

MariaDB [hellodb]> show tables;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| newstudents       |
| scores            |
| students          |
| students2         |
| teachers          |
| toc               |
+-------------------+
9 rows in set (0.00 sec)

查看修改表帮助：
MariaDB [hellodb]> help ALTER TABLE
Name: 'ALTER TABLE'
Description:
Syntax:
ALTER [ONLINE | OFFLINE] [IGNORE] TABLE tbl_name
    [alter_specification [, alter_specification] ...]
    [partition_options]

修改表：
MariaDB [hellodb]> show tables;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| scores            |
| students          |
| teachers          |
| toc               |
+-------------------+
7 rows in set (0.00 sec)

MariaDB [hellodb]> alter table students rename s1;   （修改表名为s1）
Query OK, 0 rows affected (0.00 sec)

MariaDB [hellodb]> select * from students;
ERROR 1146 (42S02): Table 'hellodb.students' doesn't exist
MariaDB [hellodb]> show tables;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| s1                |
| scores            |
| teachers          |
| toc               |
+-------------------+
7 rows in set (0.00 sec)

MariaDB [hellodb]> alter table s1 add phone varchar(11) after name; （在name字段后面添加phone字段）  ADD：添加字段   varchar可变的 字符长度为11为
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   | NULL  |  22 | M      |       2 |         3 |
|     2 | Shi Potian    | NULL  |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     | NULL  |  53 | M      |       2 |        16 |
|     4 | Ding Dian     | NULL  |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     | NULL  |  26 | M      |       3 |         1 |
|     6 | Shi Qing      | NULL  |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        | NULL  |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     | NULL  |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  | NULL  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  | NULL  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi | NULL  |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  | NULL  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  | NULL  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   | NULL  |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       | NULL  |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        | NULL  |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     | NULL  |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      | NULL  |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   | NULL  |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     | NULL  |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying | NULL  |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     | NULL  |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       | NULL  |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       | NULL  |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   | NULL  | 100 | M      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)

MariaDB [hellodb]> alter table s1 modify phone int;  （把phone的数据类型改为int）
Query OK, 25 rows affected (0.01 sec)              
Records: 25  Duplicates: 0  Warnings: 0

MariaDB [hellodb]> desc s1
    -> ;
+-----------+---------------------+------+-----+---------+----------------+
| Field     | Type                | Null | Key | Default | Extra          |
+-----------+---------------------+------+-----+---------+----------------+
| StuID     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| Name      | varchar(50)         | NO   |     | NULL    |                |
| phone     | int(11)             | YES  |     | NULL    |                |
| Age       | tinyint(3) unsigned | NO   |     | NULL    |                |
| Gender    | enum('F','M')       | NO   |     | NULL    |                |
| ClassID   | tinyint(3) unsigned | YES  |     | NULL    |                |
| TeacherID | int(10) unsigned    | YES  |     | NULL    |                |
+-----------+---------------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)

（这些就不一一解释了有兴趣的可以自己试一试）
ALTER TABLE s1 CHANGE COLUMN phone mobile char(11);
ALTER TABLE s1 DROP COLUMN mobile;
ALTER TABLE s1 character set utf8;
ALTER TABLE s1 change name name varchar(20) character set utf8;
ALTER TABLE students ADD gender ENUM('m','f');
ALETR TABLE students CHANGE id sid int UNSIGNED NOT NULL PRIMARY KEY;
ALTER TABLE students drop primary key ;
ALTER TABLE students drop primary key ;
DESC students;
ALTER TABLE students DROP age;

DML语句

DML: INSERT, DELETE, UPDATE

INSERT 语句
功能：一次插入一行或多行数据 语法

UPDATE 语句
语句用于修改表中的数据。 注意：一定要有限制条件，否则将修改所有行的指定字段

DELETE语句
语句用于删除表中的行。 注意：一定要有限制条件，否则将清空表中的所有数据
可以在不删除表的情况下删除所有的行。这意味着表的结构、属性和索引都是完整的：
DELETE FROM table_name 或者 DELETE * FROM table_name

MariaDB [hellodb]> help insert;
Name: 'INSERT'
Description:
Syntax:
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    {VALUES | VALUE} ({expr | DEFAULT},...),(...),...
    [ ON DUPLICATE KEY UPDATE
      col_name=expr
        [, col_name=expr] ... ]

Or:

INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name
    SET col_name={expr | DEFAULT}, ...
    [ ON DUPLICATE KEY UPDATE
      col_name=expr
        [, col_name=expr] ... ]

Or:

INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
    [INTO] tbl_name [(col_name,...)]
    SELECT ...
    [ ON DUPLICATE KEY UPDATE
      col_name=expr
        [, col_name=expr] ... ]

第一种语法

MariaDB [hellodb]> insert into s1 values(26,'xietingfeng',NULL,23,'M',2,1); (添加一行表记录) 
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
+-------+---------------+-------+-----+--------+---------+-----------+
26 rows in set (0.00 sec)

（如果出现这个提示可能是你少写了一个字段或者写错了）
MariaDB [hellodb]> insert into s1 values(26,'xietingfeng',23,'M',2,1);
ERROR 1136 (21S01): Column count doesn't match value count at row 1

注意：如果想省略字段名的话，只能所有字段都赋值的话可以省略，单个赋值不可以省略。
如下：挑出三个字段来赋值，必须对应的写上。
MariaDB [hellodb]> insert into s1(name,age,classid) values ('liudehua',18,1);
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
27 rows in set (0.00 sec)              （↑ 这里是因为表结构设置默认不允许为空随机分配的值）

一次行两条记录赋值：
MariaDB [hellodb]> insert into s1(name,age,gender,classid) values('mahuateng',20,'M',3),('wuyanzu',19,'M',4);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
29 rows in set (0.00 sec)

第二种语法

MariaDB [hellodb]> insert s1 set name='wuzetian',age=21,gender='F';
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
30 rows in set (0.00 sec)
(使用的不多一般使用第一种了解一下)

第三种语法

MariaDB [hellodb]> select * from teachers;
+-----+---------------+-----+--------+
| TID | Name          | Age | Gender |
+-----+---------------+-----+--------+
|   1 | Song Jiang    |  45 | M      |
|   2 | Zhang Sanfeng |  94 | M      |
|   3 | Miejue Shitai |  77 | F      |
|   4 | Lin Chaoying  |  93 | F      |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)

MariaDB [hellodb]> insert s1 (name,classid,gender,age) select name,age,gender,18 from teachers;
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
34 rows in set (0.00 sec)
（批量导入，A表参考B表的结构导入到B表中，主键必须是唯一的不可导入A表的主键）

MariaDB [hellodb]> help update;  (语句帮助)
Name: 'UPDATE'
Description:
Syntax:
Single-table syntax:

UPDATE [LOW_PRIORITY] [IGNORE] table_reference
    SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
    [WHERE where_condition]
    [ORDER BY ...]
    [LIMIT row_count]

MariaDB [hellodb]> select * from n1;  （不加指定修改会把整个列都修改）
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
34 rows in set (0.00 sec)

MariaDB [hellodb]> update n1 set teacherid=1; 
Query OK, 32 rows affected (0.00 sec)
Rows matched: 34  Changed: 32  Warnings: 0

MariaDB [hellodb]> select * from n1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         1 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         1 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |         1 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         1 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |         1 |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |         1 |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |         1 |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |         1 |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |         1 |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |         1 |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |         1 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |         1 |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |         1 |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |         1 |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |         1 |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |         1 |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |         1 |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |         1 |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |         1 |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |         1 |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |         1 |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |         1 |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |         1 |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |         1 |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |         1 |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |         1 |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |         1 |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |         1 |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |         1 |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |         1 |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |         1 |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |         1 |
+-------+---------------+-------+-----+--------+---------+-----------+
34 rows in set (0.00 sec)

避免的方法：登陆的时候加-U进入安全模式，改模式下不加where指定不能更改。
[root@centos7 ~]#mysql -U -uroot -p123456
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 11
Server version: 10.2.29-MariaDB-log MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> update n1 set teacherid=2;
ERROR 1046 (3D000): No database selected
MariaDB [(none)]> 

或者修改配文件： 在mysql下面加上这一行
[root@centos7 ~]#vim /etc/my.cnf.d/mysql-clients.cnf
[mysql]
safe-updates

MariaDB [hellodb]> update n1 set teacherid=2 where stuid=25;  （用where指定主键的某一范围修改）
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [hellodb]> select * from n1
    -> ;
+-------+---------------+-------+-----+--------+---------+-----------+
| stuid | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         1 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         1 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |         1 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         1 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |         1 |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |         1 |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |         1 |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |         1 |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |         1 |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |         1 |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |         1 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |         1 |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |         1 |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |         1 |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |         1 |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |         1 |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |         1 |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |         1 |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |         1 |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |         1 |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |         1 |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |         1 |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |         1 |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |         2 |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |         1 |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |         1 |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |         1 |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |         1 |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |         1 |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |         1 |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |         1 |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |         1 |
+-------+---------------+-------+-----+--------+---------+-----------+
34 rows in set (0.00 sec)
注意：安全模式下，这样修改表必须有主键才可以修改。

添加主键方法：
MariaDB [hellodb]> alter table n1 change stuid stuid int primary key;

MariaDB [hellodb]> delete from n1 where stuid=28;  （删除这一行）
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from n1; 
+-------+---------------+-------+-----+--------+---------+-----------+
| stuid | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+

|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |         1 |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |         2 |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |         1 |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |         1 |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |         1 |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |         1 |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |         1 |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |         1 |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |         1 |
+-------+---------------+-------+-----+--------+---------+-----------+
33 rows in set (0.00 sec)

(第二种清空数据保留表结构的方式)
MariaDB [hellodb]> delete from n1;
Query OK, 33 rows affected (0.00 sec)

MariaDB [hellodb]> select * from n1;
Empty set (0.00 sec)

MariaDB [hellodb]> desc n1;  （表结构还在）
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| stuid     | int(11)             | NO   | PRI | NULL    |       |
| Name      | varchar(50)         | NO   |     | NULL    |       |
| phone     | int(11)             | YES  |     | NULL    |       |
| Age       | tinyint(3) unsigned | NO   |     | NULL    |       |
| Gender    | enum('F','M')       | NO   |     | NULL    |       |
| ClassID   | tinyint(3) unsigned | YES  |     | NULL    |       |
| TeacherID | int(10) unsigned    | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
7 rows in set (0.00 sec)

DQL语句

查询语句：SELECT （单表和多表）

单表操作
语法：

SELECT
[ALL | DISTINCT | DISTINCTROW ]
[SQL_CACHE | SQL_NO_CACHE]
select_expr [, select_expr ...]
[FROM table_references
[WHERE where_condition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_condition]
[ORDER BY {col_name | expr | position}
[ASC | DESC], ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[FOR UPDATE | LOCK IN SHARE MODE]

说明：

字段显示可以使用别名： col1 AS alias1, col2 AS alias2, ...
WHERE子句：指明过滤条件以实现“选择”的功能：过滤条件：布尔型表达式算术操作符：+, -, *,
/, % 比较操作符：=,<=>（相等或都为空）, <>, !=(非标准SQL), >, >=, <, <= BETWEEN min_num
AND max_num IN (element1, element2, ...) IS NULL IS NOT NULL SELECT DISTINCT 去除重复
列，范例：SELECT DISTINCT gender FROM students; LIKE: % 任意长度的任意字符 _ 任意单个字
符 RLIKE：正则表达式，索引失效，不建议使用 REGEXP：匹配字符串可用正则表达式书写模式，
同上逻辑操作符：NOT，AND，OR，XOR
GROUP：根据指定的条件把查询结果进行“分组”以用于做“聚合”运算常见聚合函数：avg(), max(),
min(), count(), sum() HAVING: 对分组聚合运算后的结果指定过滤条件
ORDER BY: 根据指定的字段对查询结果进行排序升序：ASC 降序：DESC
LIMIT [[offset,]row_count]：对查询的结果进行输出行数数量限制
对查询结果中的数据请求施加“锁” FOR UPDATE: 写锁，独占或排它锁，只有一个读和写操作
LOCK IN SHARE MODE: 读锁，共享锁，同时多个读操作

范例：

TRUNCATE TABLE tbl_name;
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[SQL_CACHE | SQL_NO_CACHE]
select_expr [, select_expr ...]
[FROM table_references
[WHERE where_condition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_condition]
[ORDER BY {col_name | expr | position}
[ASC | DESC], ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[FOR UPDATE | LOCK IN SHARE MODE]
DESC students;
INSERT INTO students VALUES(1,'tom'，'m'),(2,'alice','f');
INSERT INTO students(id,name) VALUES(3,'jack'),(4,'allen');
SELECT * FROM students WHERE id < 3;
SELECT * FROM students WHERE gender='m';
SELECT * FROM students WHERE gender IS NULL;
SELECT * FROM students WHERE gender IS NOT NULL;
SELECT * FROM students ORDER BY name DESC LIMIT 2;
SELECT * FROM students ORDER BY name DESC LIMIT 1,2;
SELECT * FROM students WHERE id >=2 and id <=4
SELECT * FROM students WHERE BETWEEN 2 AND 4
SELECT * FROM students WHERE name LIKE ‘t%’
SELECT * FROM students WHERE name RLIKE '.*[lo].*';
SELECT id stuid,name as stuname FROM students

展示表里的所有内容*代表所有字段，也可以挑指定字段来显示。

MariaDB [hellodb]> select * from s1;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
.... 省略掉
+-------+---------------+-------+-----+--------+---------+-----------+
34 rows in set (0.00 sec)

甚至可以做数字运算

MariaDB [hellodb]> select 2*3;
+-----+
| 2*3 |
+-----+
|   6 |
+-----+
1 row in set (0.01 sec)

MariaDB [hellodb]> select 2*3 from teachers;  (指定一个表，表里有几行做几次数字运算和AWK很相似)
+-----+
| 2*3 |
+-----+
|   6 |
|   6 |
|   6 |
|   6 |
+-----+
4 rows in set (0.00 sec)

MariaDB [hellodb]> select * from teachers;
+-----+---------------+-----+--------+
| TID | Name          | Age | Gender |
+-----+---------------+-----+--------+
|   1 | Song Jiang    |  45 | M      |
|   2 | Zhang Sanfeng |  94 | M      |
|   3 | Miejue Shitai |  77 | F      |
|   4 | Lin Chaoying  |  93 | F      |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)

指定字段名更改加*的话不行

MariaDB [hellodb]> select '韦小宝',name from teachers;
+-----------+---------------+
| 韦小宝    | name          |
+-----------+---------------+
| 韦小宝    | Song Jiang    |
| 韦小宝    | Zhang Sanfeng |
| 韦小宝    | Miejue Shitai |
| 韦小宝    | Lin Chaoying  |
+-----------+---------------+
4 rows in set (0.00 sec)

MariaDB [hellodb]> select '韦小宝',name,age,gender from teachers;
+-----------+---------------+-----+--------+
| 韦小宝    | name          | age | gender |
+-----------+---------------+-----+--------+
| 韦小宝    | Song Jiang    |  45 | M      |
| 韦小宝    | Zhang Sanfeng |  94 | M      |
| 韦小宝    | Miejue Shitai |  77 | F      |
| 韦小宝    | Lin Chaoying  |  93 | F      |
+-----------+---------------+-----+--------+
4 rows in set (0.00 sec)

(只是显示效果更改)
MariaDB [hellodb]> select * from teachers;
+-----+---------------+-----+--------+
| TID | Name          | Age | Gender |
+-----+---------------+-----+--------+
|   1 | Song Jiang    |  45 | M      |
|   2 | Zhang Sanfeng |  94 | M      |
|   3 | Miejue Shitai |  77 | F      |
|   4 | Lin Chaoying  |  93 | F      |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)

对表头起别名显示

MariaDB [hellodb]> select '韦小宝',name as "姓名",age "年龄",gender from teachers;
+-----------+---------------+--------+--------+
| 韦小宝    | 姓名          | 年龄   | gender |
+-----------+---------------+--------+--------+
| 韦小宝    | Song Jiang    |     45 | M      |
| 韦小宝    | Zhang Sanfeng |     94 | M      |
| 韦小宝    | Miejue Shitai |     77 | F      |
| 韦小宝    | Lin Chaoying  |     93 | F      |
+-----------+---------------+--------+--------+
4 rows in set (0.01 sec)

行过滤：挑出性别不是M的 !=是非标准的SQL语法标准的为<>

MariaDB [hellodb]>  select * from s1 where gender != 'M';  (只能在MySQL数据库使用)
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
14 rows in set (0.00 sec)

MariaDB [hellodb]>  select * from s1 where gender <> 'M'; （所有SQL数据库都可以使用）
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
14 rows in set (0.00 sec)

挑出大于30的

MariaDB [hellodb]>  select * from s1 where age >=30;
+-------+--------------+-------+-----+--------+---------+-----------+
| StuID | Name         | phone | Age | Gender | ClassID | TeacherID |
+-------+--------------+-------+-----+--------+---------+-----------+
|     3 | Xie Yanke    |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian    |  NULL |  32 | M      |       4 |         4 |
|     6 | Shi Qing     |  NULL |  46 | M      |       5 |      NULL |
|    13 | Tian Boguang |  NULL |  33 | M      |       2 |      NULL |
|    25 | Sun Dasheng  |  NULL | 100 | M      |    NULL |      NULL |
+-------+--------------+-------+-----+--------+---------+-----------+
5 rows in set (0.00 sec)

挑出20到30之间的

MariaDB [hellodb]>  select * from s1 where age >=20 and age <=30;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
15 rows in set (0.00 sec)

MariaDB [hellodb]>  select * from s1 where age between 20 and 30; (结果等价于上面的)
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
15 rows in set (0.00 sec)

挑出大于30小于20的

MariaDB [hellodb]>  select * from s1 where age >=30 or age <=20;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
22 rows in set (0.00 sec)

挑出1班，3班，6班的人

MariaDB [hellodb]>  select * from s1 where classid in (1,3,6);
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
14 rows in set (0.00 sec)

查看班级id为空的

MariaDB [hellodb]>  select * from s1 where classid is null;
+-------+-------------+-------+-----+--------+---------+-----------+
| StuID | Name        | phone | Age | Gender | ClassID | TeacherID |
+-------+-------------+-------+-----+--------+---------+-----------+
|    24 | Xu Xian     |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng |  NULL | 100 | M      |    NULL |      NULL |
|    30 | wuzetian    |  NULL |  21 | F      |    NULL |      NULL |
+-------+-------------+-------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)

查看班级id不为空的

MariaDB [hellodb]>  select * from s1 where classid is not null;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
31 rows in set (0.00 sec)

添加空值必须指定为null

MariaDB [hellodb]> insert students (name,age,gender,classid)values('abc',20,'M',null);
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]>  select * from s1 where classid is null;
+-------+-------------+-------+-----+--------+---------+-----------+
| StuID | Name        | phone | Age | Gender | ClassID | TeacherID |
+-------+-------------+-------+-----+--------+---------+-----------+
|    24 | Xu Xian     |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng |  NULL | 100 | M      |    NULL |      NULL |
|    30 | wuzetian    |  NULL |  21 | F      |    NULL |      NULL |
|    38 | abc         |  NULL |  20 | M      |    NULL |      NULL |
+-------+-------------+-------+-----+--------+---------+-----------+
4 rows in set (0.00 sec)

去除calssid重复的列

MariaDB [hellodb]> select distinct classid from s1;
+---------+
| classid |
+---------+
|       2 |
|       1 |
|       4 |
|       3 |
|       5 |
|       7 |
|       6 |
|    NULL |
|      45 |
|      94 |
|      77 |
|      93 |
+---------+
12 rows in set (0.00 sec)

where语句模糊搜索匹配w开头的行（like）

MariaDB [hellodb]> select * from s1 where name like 'w%';
+-------+--------------+-------+-----+--------+---------+-----------+
| StuID | Name         | phone | Age | Gender | ClassID | TeacherID |
+-------+--------------+-------+-----+--------+---------+-----------+
|    12 | Wen Qingqing |  NULL |  19 | F      |       1 |      NULL |
|    29 | wuyanzu      |  NULL |  19 | M      |       4 |      NULL |
|    30 | wuzetian     |  NULL |  21 | F      |    NULL |      NULL |
+-------+--------------+-------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)

MariaDB [hellodb]> select * from s1 where name like '%w%'; (这个显示为包含w的行)
+-------+--------------+-------+-----+--------+---------+-----------+
| StuID | Name         | phone | Age | Gender | ClassID | TeacherID |
+-------+--------------+-------+-----+--------+---------+-----------+
|    12 | Wen Qingqing |  NULL |  19 | F      |       1 |      NULL |
|    14 | Lu Wushuang  |  NULL |  17 | F      |       3 |      NULL |
|    29 | wuyanzu      |  NULL |  19 | M      |       4 |      NULL |
|    30 | wuzetian     |  NULL |  21 | F      |    NULL |      NULL |
+-------+--------------+-------+-----+--------+---------+-----------+
4 rows in set (0.00 sec)

查看字符串数量为三的：用三个下划线来表示

MariaDB [hellodb]> select * from s1 where name like '___';
+-------+------+-------+-----+--------+---------+-----------+
| StuID | Name | phone | Age | Gender | ClassID | TeacherID |
+-------+------+-------+-----+--------+---------+-----------+
|    38 | abc  |  NULL |  20 | M      |    NULL |      NULL |
+-------+------+-------+-----+--------+---------+-----------+
1 row in set (0.00 sec)

rlike '^#'等价于like '#%'

MariaDB [hellodb]> select * from s1 where name rlike '^x';
+-------+-------------+-------+-----+--------+---------+-----------+
| StuID | Name        | phone | Age | Gender | ClassID | TeacherID |
+-------+-------------+-------+-----+--------+---------+-----------+
|     3 | Xie Yanke   |  NULL |  53 | M      |       2 |        16 |
|     7 | Xi Ren      |  NULL |  19 | F      |       3 |      NULL |
|    16 | Xu Zhu      |  NULL |  21 | M      |       1 |      NULL |
|    19 | Xue Baochai |  NULL |  18 | F      |       6 |      NULL |
|    22 | Xiao Qiao   |  NULL |  20 | F      |       1 |      NULL |
|    24 | Xu Xian     |  NULL |  27 | M      |    NULL |      NULL |
|    26 | xietingfeng |  NULL |  23 | M      |       2 |         1 |
+-------+-------------+-------+-----+--------+---------+-----------+
7 rows in set (0.00 sec)

MariaDB [hellodb]> select * from s1 where name like 'x%';
+-------+-------------+-------+-----+--------+---------+-----------+
| StuID | Name        | phone | Age | Gender | ClassID | TeacherID |
+-------+-------------+-------+-----+--------+---------+-----------+
|     3 | Xie Yanke   |  NULL |  53 | M      |       2 |        16 |
|     7 | Xi Ren      |  NULL |  19 | F      |       3 |      NULL |
|    16 | Xu Zhu      |  NULL |  21 | M      |       1 |      NULL |
|    19 | Xue Baochai |  NULL |  18 | F      |       6 |      NULL |
|    22 | Xiao Qiao   |  NULL |  20 | F      |       1 |      NULL |
|    24 | Xu Xian     |  NULL |  27 | M      |    NULL |      NULL |
|    26 | xietingfeng |  NULL |  23 | M      |       2 |         1 |
+-------+-------------+-------+-----+--------+---------+-----------+
7 rows in set (0.00 sec)

建立一个表存储用户和密码

MariaDB [hellodb]> create table user (id int primary key,name varchar(10) not null,password varchar(30) not null);
Query OK, 0 rows affected (0.00 sec)

MariaDB [hellodb]> insert user value(1,'alice','centos');
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> insert user value(2,'bob','linux');
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from user;
+----+-------+----------+
| id | name  | password |
+----+-------+----------+
|  1 | alice | centos   |
|  2 | bob   | linux    |
+----+-------+----------+
2 rows in set (0.00 sec)

使用SQL注入（SQL注入漏洞）

MariaDB [hellodb]> select * from user where name='alice' and password='tuituibang'; (密码不对应返回空值)
Empty set (0.00 sec)

MariaDB [hellodb]> select * from user where name='alice'; -- ' and password='tuituibang'; （使用特殊符号密码不对应也可以查看）
+----+-------+----------+
| id | name  | password |
+----+-------+----------+
|  1 | alice | centos   |
+----+-------+----------+
1 row in set (0.00 sec)

构建一个复杂密码用户结果也一样

MariaDB [hellodb]> insert user values(3,'admin','p@ssw)rd!');
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from user;
+----+-------+-----------+
| id | name  | password  |
+----+-------+-----------+
|  1 | alice | centos    |
|  2 | bob   | linux     |
|  3 | admin | p@ssw)rd! |
+----+-------+-----------+
3 rows in set (0.00 sec)

MariaDB [hellodb]> select * from user where name='admin'; -- ' and password='tuituibang';  （这里的引号数量是单数）
+----+-------+-----------+
| id | name  | password  |
+----+-------+-----------+
|  3 | admin | p@ssw)rd! |
+----+-------+-----------+
1 row in set (0.00 sec)

在登陆的时候构建一个奇怪的密码也可以登录 （注意这里的引号数量必须是成对的）
MariaDB [hellodb]> select * from user where name='admin' and password=''  or '1=1';
+----+-------+-----------+
| id | name  | password  |
+----+-------+-----------+
|  1 | alice | centos    |
|  2 | bob   | linux     |
|  3 | admin | p@ssw)rd! |
+----+-------+-----------+
3 rows in set, 5 warnings (0.00 sec)

MariaDB [hellodb]> select * from user where name='admin' and password=''  or '1'='1'; 
+----+-------+-----------+
| id | name  | password  |
+----+-------+-----------+
|  1 | alice | centos    |
|  2 | bob   | linux     |
|  3 | admin | p@ssw)rd! |
+----+-------+-----------+
3 rows in set (0.00 sec)

单表操作的分组统计

SQL的聚合函数

AVG(column)             返回某列的平均值
BINARY_CHECKSUM  
CHECKSUM     
CHECKSUM_AGG     
COUNT(column)           返回某列的行数（不包括NULL值）
COUNT(*)                返回被选行数（函数返回在给定的选择中被选的行数）
COUNT(DISTINCT column)  返回相异结果的数目
FIRST(column)           返回在指定的域中第一个记录的值（SQLServer2000 不支持）
LAST(column)            返回在指定的域中最后一个记录的值（SQLServer2000 不支持）
MAX(column)             返回某列的最高值
MIN(column)             返回某列的最低值
STDEV(column)    
STDEVP(column)   
SUM(column)             返回某列的总和
VAR(column)  
VARP(column)

count 统计列的行数

MariaDB [hellodb]> select count(stuid) from s1;
+--------------+
| count(stuid) |
+--------------+
|           35 |
+--------------+
1 row in set (0.00 sec)

MariaDB [hellodb]> select count(classid) from s1;
+----------------+
| count(classid) |
+----------------+
|             31 |        （有空值不统计）
+----------------+
1 row in set (0.00 sec)


MariaDB [hellodb]> select count(1) from s1; （跟count（主键）一样，只扫描主键）
+----------+
| count(1) |
+----------+
|       35 |
+----------+
1 row in set (0.00 sec)

MariaDB [hellodb]> select count(*) from s1; (和count（主键）使用方式一样，但是在性能上有略微的区别，mysql对前者做了优化。)
+----------+
| count(*) |
+----------+
|       35 |
+----------+
1 row in set (0.00 sec)

使用GROUP BY （根据指定的条件把查询结果进行“分组”以用于做“聚合”运算常见聚合函数）统计男生女生分别多少个

MariaDB [hellodb]> select gender,count(*) from s1 group by gender ;
+--------+----------+
| gender | count(*) |
+--------+----------+
| F      |       14 |
| M      |       21 |
+--------+----------+
2 rows in set (0.00 sec)

(起个别名)
MariaDB [hellodb]> select gender 性别,count(*) 人数 from s1 group by gender ;
+--------+--------+
| 性别   | 人数   |
+--------+--------+
| F      |     14 |
| M      |     21 |
+--------+--------+
2 rows in set (0.00 sec)

AVG 统计平均年龄

MariaDB [hellodb]> select gender,avg(age) from s1 group by gender;
+--------+----------+
| gender | avg(age) |
+--------+----------+
| F      |  18.9286 |
| M      |  29.1905 |
+--------+----------+
2 rows in set (0.00 sec)

max 统计男女生最大的年龄

MariaDB [hellodb]> select gender,avg(age),max(age) from s1 group by gender;
+--------+----------+----------+
| gender | avg(age) | max(age) |
+--------+----------+----------+
| F      |  18.9286 |       22 |
| M      |  29.1905 |      100 |
+--------+----------+----------+
2 rows in set (0.00 sec)

分组之后过滤行的话不能加where 要加having (在group by后面使用)

MariaDB [hellodb]> select classid,gender,avg(age) from s1 group by classid,gender;
+---------+--------+----------+              (过滤掉空的行)
| classid | gender | avg(age) |
+---------+--------+----------+
|    NULL | F      |  21.0000 |
|    NULL | M      |  49.0000 |
|       1 | F      |  19.0000 |
|       1 | M      |  21.5000 |
|       2 | M      |  32.7500 |
|       3 | F      |  18.3333 |
|       3 | M      |  23.0000 |
|       4 | M      |  23.6000 |
|       5 | M      |  46.0000 |
|       6 | F      |  20.0000 |
|       6 | M      |  23.0000 |
|       7 | F      |  18.0000 |
|       7 | M      |  23.0000 |
|      45 | M      |  18.0000 |
|      77 | F      |  18.0000 |
|      93 | F      |  18.0000 |
|      94 | M      |  18.0000 |
+---------+--------+----------+
17 rows in set (0.00 sec)

MariaDB [hellodb]> select classid,gender,avg(age) from s1 group by classid,gender where classid is not null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'where classid is not null' at line 1
MariaDB [hellodb]> select classid,gender,avg(age) from s1 group by classid,gender having classid is not null;  （HAVING: 对分组聚合运算后的结果指定过滤条件）
+---------+--------+----------+
| classid | gender | avg(age) |
+---------+--------+----------+
|       1 | F      |  19.0000 |
|       1 | M      |  21.5000 |
|       2 | M      |  32.7500 |
|       3 | F      |  18.3333 |
|       3 | M      |  23.0000 |
|       4 | M      |  23.6000 |
|       5 | M      |  46.0000 |
|       6 | F      |  20.0000 |
|       6 | M      |  23.0000 |
|       7 | F      |  18.0000 |
|       7 | M      |  23.0000 |
|      45 | M      |  18.0000 |
|      77 | F      |  18.0000 |
|      93 | F      |  18.0000 |
|      94 | M      |  18.0000 |
+---------+--------+----------+
15 rows in set (0.00 sec)

分组前可以使用where过滤（在group by之前添加）

MariaDB [hellodb]> select classid,gender,avg(age) from s1 where classid is not null group by classid,gender;
+---------+--------+----------+
| classid | gender | avg(age) |
+---------+--------+----------+
|       1 | F      |  19.0000 |
|       1 | M      |  21.5000 |
|       2 | M      |  32.7500 |
|       3 | F      |  18.3333 |
|       3 | M      |  23.0000 |
|       4 | M      |  23.6000 |
|       5 | M      |  46.0000 |
|       6 | F      |  20.0000 |
|       6 | M      |  23.0000 |
|       7 | F      |  18.0000 |
|       7 | M      |  23.0000 |
|      45 | M      |  18.0000 |
|      77 | F      |  18.0000 |
|      93 | F      |  18.0000 |
|      94 | M      |  18.0000 |
+---------+--------+----------+
15 rows in set (0.00 sec)

ORDER BY 排序单字段排序：默认按照字母正序排序

MariaDB [hellodb]> select * from s1 order by name;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    38 | abc           |  NULL |  20 | M      |    NULL |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
35 rows in set (0.00 sec)

倒序排序

MariaDB [hellodb]> select * from s1 order by name desc;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    38 | abc           |  NULL |  20 | M      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
35 rows in set (0.00 sec)

指定asc排序（asc：以数字大小从小到大排序，空值默认为0排在最前面）

MariaDB [hellodb]> select * from s1 order by classid asc;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    38 | abc           |  NULL |  20 | M      |    NULL |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
35 rows in set (0.00 sec)

还是按照数字从小到大排序但是空值排在最后面


MariaDB [hellodb]> select * from s1 order by -classid desc;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|    38 | abc           |  NULL |  20 | M      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
35 rows in set (0.00 sec)

多字段排序（先对性别排序在对年龄排序）

MariaDB [hellodb]> select * from s1 order by gender , age;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    38 | abc           |  NULL |  20 | M      |    NULL |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
35 rows in set (0.00 sec)

先对性别降序排序在对年龄正序排序。

MariaDB [hellodb]> select * from s1 order by gender desc,age asc;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    38 | abc           |  NULL |  20 | M      |    NULL |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|    24 | Xu Xian       |  NULL |  27 | M      |    NULL |      NULL |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|    25 | Sun Dasheng   |  NULL | 100 | M      |    NULL |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    30 | wuzetian      |  NULL |  21 | F      |    NULL |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
35 rows in set (0.00 sec)

多字段排序之后无法在做过滤 (但是排序前可以)

MariaDB [hellodb]> select * from s1 order by gender desc,age asc where classid is not null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'where classid is not null' at line 1
MariaDB [hellodb]> select * from s1 order by gender desc,age asc having classid is not null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'having classid is not null' at line 1
MariaDB [hellodb]> select * from s1 order by gender desc,age asc on classid is not null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'on classid is not null' at line 1

(先过滤在排序)
MariaDB [hellodb]> select * from s1 where classid is not null order by gender desc,age asc;  （和group by 一样还是在前面加）
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|    28 | mahuateng     |  NULL |  20 | M      |       3 |      NULL |
|    16 | Xu Zhu        |  NULL |  21 | M      |       1 |      NULL |
|     1 | Shi Zhongyu   |  NULL |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  NULL |  22 | M      |       1 |         7 |
|    23 | Ma Chao       |  NULL |  23 | M      |       4 |      NULL |
|    26 | xietingfeng   |  NULL |  23 | M      |       2 |         1 |
|    11 | Yuan Chengzhi |  NULL |  23 | M      |       6 |      NULL |
|    18 | Hua Rong      |  NULL |  23 | M      |       7 |      NULL |
|    17 | Lin Chong     |  NULL |  25 | M      |       4 |      NULL |
|     5 | Yu Yutong     |  NULL |  26 | M      |       3 |         1 |
|     4 | Ding Dian     |  NULL |  32 | M      |       4 |         4 |
|    13 | Tian Boguang  |  NULL |  33 | M      |       2 |      NULL |
|     6 | Shi Qing      |  NULL |  46 | M      |       5 |      NULL |
|     3 | Xie Yanke     |  NULL |  53 | M      |       2 |        16 |
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    10 | Yue Lingshan  |  NULL |  19 | F      |       3 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    22 | Xiao Qiao     |  NULL |  20 | F      |       1 |      NULL |
|     9 | Ren Yingying  |  NULL |  20 | F      |       6 |      NULL |
|    21 | Huang Yueying |  NULL |  22 | F      |       6 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
31 rows in set (0.00 sec)

后面的分组字段对调位置的话显示效果不一样但是数据还是一样的。(排序逻辑不一样)

MariaDB [hellodb]> select gender,classid,avg(age) from s1 where classid is not null group by gender,classid;
+--------+---------+----------+
| gender | classid | avg(age) |
+--------+---------+----------+
| F      |       1 |  19.0000 |
| F      |       3 |  18.3333 |
| F      |       6 |  20.0000 |
| F      |       7 |  18.0000 |
| F      |      77 |  18.0000 |
| F      |      93 |  18.0000 |
| M      |       1 |  21.5000 |
| M      |       2 |  32.7500 |
| M      |       3 |  23.0000 |
| M      |       4 |  23.6000 |
| M      |       5 |  46.0000 |
| M      |       6 |  23.0000 |
| M      |       7 |  23.0000 |
| M      |      45 |  18.0000 |
| M      |      94 |  18.0000 |
+--------+---------+----------+
15 rows in set (0.00 sec)

MariaDB [hellodb]> select gender,classid,avg(age) from s1 where classid is not null group by classid,gender;
+--------+---------+----------+
| gender | classid | avg(age) |
+--------+---------+----------+
| F      |       1 |  19.0000 |
| M      |       1 |  21.5000 |
| M      |       2 |  32.7500 |
| F      |       3 |  18.3333 |
| M      |       3 |  23.0000 |
| M      |       4 |  23.6000 |
| M      |       5 |  46.0000 |
| F      |       6 |  20.0000 |
| M      |       6 |  23.0000 |
| F      |       7 |  18.0000 |
| M      |       7 |  23.0000 |
| M      |      45 |  18.0000 |
| F      |      77 |  18.0000 |
| F      |      93 |  18.0000 |
| M      |      94 |  18.0000 |
+--------+---------+----------+
15 rows in set (0.00 sec)

LIMIT [[offset,]row_count]：对查询的结果进行输出行数数量限制
正序排序取岁数最小的前10名 limit 作为限制条件

MariaDB [hellodb]> select * from s1 order by age limit 10;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    14 | Lu Wushuang   |  NULL |  17 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  NULL |  17 | F      |       7 |      NULL |
|    32 | Zhang Sanfeng |  NULL |  18 | M      |      94 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
10 rows in set (0.00 sec)

跳过前三行取岁数最小的前10名

MariaDB [hellodb]> select * from s1 order by age limit 3,10;
+-------+---------------+-------+-----+--------+---------+-----------+
| StuID | Name          | phone | Age | Gender | ClassID | TeacherID |
+-------+---------------+-------+-----+--------+---------+-----------+
|    33 | Miejue Shitai |  NULL |  18 | F      |      77 |      NULL |
|    19 | Xue Baochai   |  NULL |  18 | F      |       6 |      NULL |
|    31 | Song Jiang    |  NULL |  18 | M      |      45 |      NULL |
|    27 | liudehua      |  NULL |  18 | F      |       1 |      NULL |
|    34 | Lin Chaoying  |  NULL |  18 | F      |      93 |      NULL |
|    20 | Diao Chan     |  NULL |  19 | F      |       7 |      NULL |
|    12 | Wen Qingqing  |  NULL |  19 | F      |       1 |      NULL |
|    15 | Duan Yu       |  NULL |  19 | M      |       4 |      NULL |
|     7 | Xi Ren        |  NULL |  19 | F      |       3 |      NULL |
|    29 | wuyanzu       |  NULL |  19 | M      |       4 |      NULL |
+-------+---------------+-------+-----+--------+---------+-----------+
10 rows in set (0.00 sec)

去重之后在做过滤（这样取出来的数值比较准确）

MariaDB [hellodb]> select distinct age from s1 order by age limit 3; (去重取前三个)
+-----+
| age |
+-----+
|  17 |
|  18 |
|  19 |
+-----+
3 rows in set (0.00 sec)

MariaDB [hellodb]> select distinct age from s1 order by age limit 3,6; （去重跳过前3行取6行）
+-----+
| age |
+-----+
|  20 |
|  21 |
|  22 |
|  23 |
|  25 |
|  26 |
+-----+
6 rows in set (0.00 sec)

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