MySQL使用自定义变量模拟分析函数
MySQL使用自定义变量模拟分析函数
MySQL使用自定义变量模拟分析函数，初始化实验结构和数据：
create table test(
tid varchar(32) primary key,
stat int not null,
createtime timestamp not null
);
INSERT INTO test (tid,stat,createtime) VALUES ('ac551ad7ba3f9067b19ac8bb20caca6d',-1,'2014-08-20 10:01:09');
INSERT INTO test (tid,stat,createtime) VALUES ('dc9a4438e577f4b08f7033a305544d47',-1,'2014-08-20 10:00:19');
INSERT INTO test (tid,stat,createtime) VALUES ('23055228532bbba5a68d6ada11bcf33f',-1,'2014-08-20 09:58:32');
INSERT INTO test (tid,stat,createtime) VALUES ('5711ee1610d07a55e64c7948667de6e8',-1,'2014-08-20 09:58:09');
INSERT INTO test (tid,stat,createtime) VALUES ('035e06d8afd681a9904bd74e9860f8cb',-1,'2014-08-20 09:57:52');
INSERT INTO test (tid,stat,createtime) VALUES ('3890efc08f37fa489a4e130cb04f71ac',-1,'2014-08-20 09:57:48');
INSERT INTO test (tid,stat,createtime) VALUES ('1b6ed9db663dae470b45c722a61d08b0',-1,'2014-08-20 09:56:40');
INSERT INTO test (tid,stat,createtime) VALUES ('8fb3409015e6b2cf85ba6ee90f15b58f',-1,'2014-08-20 09:54:40');
INSERT INTO test (tid,stat,createtime) VALUES ('0badb1f4c2b1a89f1c473b992183add3',-1,'2014-08-20 09:54:33');
INSERT INTO test (tid,stat,createtime) VALUES ('89b8af5eb473b2d4f50dd9e10773a9cc',-1,'2014-08-20 09:53:54');
INSERT INTO test (tid,stat,createtime) VALUES ('77923a7397110224b5f94e7d0bd297de',2,'2014-08-19 17:13:17');
INSERT INTO test (tid,stat,createtime) VALUES ('0df1da77cfdbe64edcd4d645197174af',2,'2014-08-19 12:20:21');
INSERT INTO test (tid,stat,createtime) VALUES ('43daef6bfbc46dbfdbb97e74173dab30',2,'2014-08-19 09:54:08');
INSERT INTO test (tid,stat,createtime) VALUES ('d5d12c510391314f48054c6c9ab9535c',2,'2014-08-19 09:23:41');
INSERT INTO test (tid,stat,createtime) VALUES ('f7c123143752498b7c9a226a9583ae49',2,'2014-08-19 01:14:21');
INSERT INTO test (tid,stat,createtime) VALUES ('da6a9a78897a42ae0a565cd0fabd76bb',2,'2014-08-18 21:59:46');
INSERT INTO test (tid,stat,createtime) VALUES ('9cd3f83ab04120504a880523702491d7',2,'2014-08-18 16:26:30');
INSERT INTO test (tid,stat,createtime) VALUES ('4dfa129ba64e7062afa37e56bb9632de',2,'2014-08-18 14:32:41');
INSERT INTO test (tid,stat,createtime) VALUES ('a9a731870e1c02278c22ce1ab36fa43c',2,'2014-08-18 14:31:26');
INSERT INTO test (tid,stat,createtime) VALUES ('97f39d2a1e519f99e602e72cfc45fe0c',2,'2014-08-17 11:47:52');
INSERT INTO test (tid,stat,createtime) VALUES ('31ba95265a96971221ddf9320c79eed8',3,'2014-08-20 02:08:50');
INSERT INTO test (tid,stat,createtime) VALUES ('060d92222edcb6f583cb4cd0244aadc0',3,'2014-08-20 02:05:54');
INSERT INTO test (tid,stat,createtime) VALUES ('7d3eb4ea201906b08e961b9fe7726fd4',3,'2014-08-20 02:00:11');
INSERT INTO test (tid,stat,createtime) VALUES ('c633bc16cb8c3bb4ffa7f00682701b92',3,'2014-08-20 01:54:22');
INSERT INTO test (tid,stat,createtime) VALUES ('e43bb7e7274259712b389e3feabc068f',3,'2014-08-20 01:49:36');
INSERT INTO test (tid,stat,createtime) VALUES ('bdabf3d80fb097222112cb30cdc48117',3,'2014-08-20 01:48:48');
INSERT INTO test (tid,stat,createtime) VALUES ('170e2bdc11d517a56b7ce23d85633e42',3,'2014-08-20 01:46:56');
INSERT INTO test (tid,stat,createtime) VALUES ('7e79f6065ae8bb215cee43a4efbcd852',3,'2014-08-20 01:44:17');
INSERT INTO test (tid,stat,createtime) VALUES ('04728676e3305de05a18333ddfc76c01',3,'2014-08-20 01:39:05');
INSERT INTO test (tid,stat,createtime) VALUES ('d987176d350d4fefcc92b9a7ebb4f288',3,'2014-08-20 01:35:52');
COMMIT;
要求:
Stat表示状态,Createtime表示创建时间
查询每种状态最近3个记录的内容
这个需求似曾相识
但是这回是MySQL数据库了。他没有分析函数，只能用自定义变量模拟
test表的stat字段为分组标识
gid是上一个记录的分组标识,
cgid是当前记录的分组标识,
如果gid和cgid不等，说明分组标识已经变了,Rank排序重置
这样Rank是按照每个分组进行自增的,取前N个记录就可以了。
SELECT t3.tid, t3.stat, t3.createtime
FROM (SELECT @gid := @cgid, @cgid := t1.stat, if(@gid = @cgid, @rank := @rank + 1, @rank := 1) AS rank, t1.*
FROM (SELECT *
FROM test
ORDER BY stat, createtime DESC
) t1, (SELECT @gid := 1, @cgid := 1, @rank := 1) t2
) t3
WHERE t3.rank <= 3;
结果:
MySQL实现每个分组随机抽取N个记录的功能
(前面链接中的功能)
SELECT t3.tid, t3.stat, t3.createtime
FROM (SELECT @gid := @cgid, @cgid := t1.stat, if(@gid = @cgid, @rank := @rank + 1, @rank := 1) AS rank, t1.*
FROM (SELECT *
FROM test
ORDER BY stat, rand()
) t1, (SELECT @gid := 1, @cgid := 1, @rank := 1) t2
) t3
WHERE t3.rank <= 3;
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上面这些都是使用MySQL自定义变量模拟分析函数,
使用传统的相关子查询也可以实现这个功能,只是效率低
SELECT t1.tid, t1.stat, t1.createtime
FROM test t1
WHERE (
SELECT COUNT(*)
FROM test t2
WHERE t2.stat = t1.stat
AND t1.createtime < t2.createtime
) < 3
ORDER BY stat, createtime DESC
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