MySQL使用自訂變數模擬分析函數

MySQL使用自訂變數模擬分析函數

MySQL使用自訂變數模擬分析函數，初始化實驗結構和資料：

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表示狀態,Createtime表示建立時間
 查詢每種狀態最近3個記錄的內容
 
這個需求似曾相識
 
但是這回是MySQL資料庫了。他沒有分析函數，只能用自訂變數類比
 test表的stat欄位為分組標識
 gid是上一個記錄的分組標識,
 cgid是目前記錄的分組標識,
 如果gid和cgid不等，說明分組標識已經變了,Rank排序重設
 這樣Rank是按照每個分組進行自增的,取前N個記錄就可以了。
 

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;
 結果:

MySQL實現每個分組隨機抽取N個記錄的功能
 (前面連結中的功能)
 
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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上面這些都是使用MySQL自訂變數模擬分析函數,
 使用傳統的相互關聯的子查詢也可以實現這個功能,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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