Oracle CONNECT BY的用法

文章由LinuxBoy分享于2019-03-30 04:03:17热评（348）

Oracle CONNECT BY的用法

Oracle中可以通过START WITH . . . CONNECT BY . . .子句来实现SQL的层次查询.
自从Oracle 9i开始，可以通过 SYS_CONNECT_BY_PATH 函数实现将父节点到当前行内容以“path”或者层次元素列表的形式显示出来。

自从Oracle 10g 中，还有其他更多关于层次查询的新特性。例如，有的时候用户更关心的是每个层次分支中等级最低的内容。

那么你就可以利用伪列函数CONNECT_BY_ISLEAF来判断当前行是不是叶子。如果是叶子就会在伪列中显示“1”，
如果不是叶子而是一个分支（例如当前内容是其他行的父亲）就显示“0”。

在Oracle 10g 之前的版本中，如果在你的树中出现了环状循环（如一个孩子节点引用一个父亲节点），
Oracle 就会报出一个错误提示：“ ORA-01436: CONNECT BY loop in user data”。如果不删掉对父亲的引用就无法执行查询操作。
而在 Oracle 10g 中，只要指定“NOCYCLE”就可以进行任意的查询操作。与这个关键字相关的还有一个伪列——CONNECT_BY_ISCYCLE，
如果在当前行中引用了某个父亲节点的内容并在树中出现了循环，那么该行的伪列中就会显示“1”，否则就显示“0”。

The start with .. connect by clause can be used to select data that has a hierarchical relationship
(usually some sort of parent->child, boss->employee or thing->parts).
It is also being used when an sql execution plan is explained.

syntax:
select ... [start with initial-condition] connect by [nocycle] recurse-condition

level
With level it is possible to show the level in the hierarchical relation of all the data.

--oracle 9i
sys_connect_by_path
With sys_connect_by_path it is possible to show the entire path from the top level down to the 'actual' child.

--oracle 10g
connect_by_root
connect_by_root is a new operator that comes with Oracle 10g and enhances the ability to perform hierarchical queries.
connect_by_is_leaf
connect_by_isleaf is a new operator that comes with Oracle 10g and enhances the ability to perform hierarchical queries.
connect_by_iscycle
connect_by_is_cycle is a new operator that comes with Oracle 10g and enhances the ability to perform hierarchical queries.

下面举例说明：
[例1]
创建一个部门表，这个表有4个字段，分别对应部门ID，部门名称，上级部门ID ，本部门直属员工人数。
create table DEP
(
DEPID      number(10) not null,
DEPNAME    varchar2(32),
UPPERDEPID number(10),
NUMOFEMP   number(10)
) ;
下面插入一些数据，结果如下：

hr@MYTEST2> select * from dep;
DEPID DEPNAME UPPERDEPID NUMOFEMP
---------- --------------- ---------- ----------
0 Dev Center 2
1 DevA 0 3
2 DevA Team1 1 10
3 DevA Team2 1 8
4 DevB 0 2
5 DevB Team1 4 12
6 Test Center 1
7 Test Team1 6 5
8 Test Team2 6 5
9 rows selected.

现在根据“CONNECT BY”来实现树状查询：

column depname format a15;
column rootdep format a15;
column path format a30;
select rpad(' ', 2*(level-1), '-') || depname "DEPNAME",
connect_by_root depname "ROOTDEP",
connect_by_isleaf "ISLEAF",
level,
sys_connect_by_path(depname, '/') "PATH"
from dep
start with upperdepid is null
connect by prior depid = upperdepid
/

结果如下：

DEPNAME ROOTDEP ISLEAF LEVEL PATH
--------------- --------------- ---------- ---------- ---------------------------
Dev Center Dev Center 0 1 /Dev Center
-DevA Dev Center 0 2 /Dev Center/DevA
---DevA Team1 Dev Center 1 3 /Dev Center/DevA/DevA Team1
---DevA Team2 Dev Center 1 3 /Dev Center/DevA/DevA Team2
-DevB Dev Center 0 2 /Dev Center/DevB
---DevB Team1 Dev Center 1 3 /Dev Center/DevB/DevB Team1
Test Center Test Center 0 1 /Test Center
-Test Team1 Test Center 1 2 /Test Center/Test Team1
-Test Team2 Test Center 1 2 /Test Center/Test Team2

下面计算Dev Center和Test Center部门的总人数：

select ROOTDEPID, sum(numofemp) "TOTALEMP"
from (select connect_by_root depid "ROOTDEPID", numofemp from dep
start with upperdepid is null
connect by prior depid = upperdepid)
group by ROOTDEPID
/

结果如下：

ROOTDEPID TOTALEMP
--------- ----------
6 11
0 37

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