MySQL学习笔记：三种组内排序方法

　　由于MySQ没有提供像Oracle的dense_rank()或者row_number() over(partition by)等函数，来实现组内排序，想实现这个功能，还是得自己想想办法，最终通过创建行号实现。

方法一：

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　　1.建立测试表

# 建表 DROP TABLE test; CREATE TABLE test ( myNAME VARCHAR (10), name1 VARCHAR (10), count1 BIGINT );

　　2.删除原有数据

# 删除原有数据 DELETE FROM test; SELECT * FROM test;

　　3.插入数据

# 插入数据 INSERT INTO test VALUES('1','a',2); INSERT INTO test VALUES('1','b',1);; INSERT INTO test VALUES('1','c',4); INSERT INTO test VALUES('1','d',5); INSERT INTO test VALUES('1','e',7); INSERT INTO test VALUES('1','f',8); INSERT INTO test VALUES('2','g',9); INSERT INTO test VALUES('2','h',0); INSERT INTO test VALUES('2','i',21); INSERT INTO test VALUES('2','j',3); INSERT INTO test VALUES('2','k',4); INSERT INTO test VALUES('2','1',56); INSERT INTO test VALUES('3','m',67); INSERT INTO test VALUES('3','n',89); INSERT INTO test VALUES('3','o',12); INSERT INTO test VALUES('3','p',22); INSERT INTO test VALUES('3','q',23); INSERT INTO test VALUES('3','r',42); INSERT INTO test VALUES('3','s',26);

　　4.查询

# 查询 SELECT * FROM test;

　　5.构造行号rownum

#---- 构造rownum ----
SELECT b.myNAME, b.name1, b.count1, IF(@name = b.myNAME, @rank := @rank + 1, @rank := 1) -- 3.判断name是否等于此行的myNAME
    AS rank,   -- 如果是，则rank自增；如果不是，则初始化rank=1
    @name := b.myNAME FROM (SELECT myNAME, name1, count1 FROM test ORDER BY myNAME ASC, count1 DESC) b,  -- 1.先进行子查询 排序
(SELECT @rownum := 0, @name := NULL, -- 2.初始化rownum=0，name=NULL, rank=0
    @rank := 0) a;

　　6.筛选组内前五（即行数小于等于5的行）

SELECT myNAME, name1, count1, rank FROM (SELECT b.myNAME, b.name1, b.count1, IF(@name = b.myNAME, @rank := @rank + 1, @rank := 1) -- 3.判断name是否等于此行的myNAME
    AS rank,   #-- 如果是，则rank自增；如果不是，则初始化rank=1
    @name := b.myNAME FROM (SELECT myNAME, name1, count1 FROM test ORDER BY myNAME ASC, count1 DESC) b,  -- 1.先进行子查询 排序
(SELECT @rownum := 0, @name := NULL, -- 2.初始化rownum=0，name=NULL, rank=0
    @rank := 0) a) result WHERE rank <=5;

 

方法二：

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　　1.关联

SELECT * 
FROM test a JOIN test b ON a.`myNAME` = b.`myNAME` AND a.`count1` <= b.`count1`;

　　2.排序

SELECT * 
FROM test a JOIN test b ON a.`myNAME` = b.`myNAME` AND a.`count1` <= b.`count1` ORDER BY a.`myNAME` ASC, a.`count1` DESC;

　　3.最终实现

SELECT a.myNAME,a.name1,a.count1,COUNT(*) AS rank FROM test a JOIN test b ON a.`myNAME` = b.`myNAME` AND a.`count1` <= b.`count1` GROUP BY a.`myNAME`,a.`name1`,a.`count1` ORDER BY a.`myNAME` ASC, a.`count1` DESC;

 

方法三：

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　　其实方法三只是方法二的补充，针对有数据相等的情况。

SELECT a.myNAME, a.name1, a.count1, COUNT(1) AS rank FROM test AS a, test AS b WHERE a.myNAME = b.myNAME AND a.count1 < b.count1 OR (a.count1 = b.count1 AND a.myNAME <= b.myNAME) GROUP BY a.myNAME, a.name1, a.count1 ORDER BY a.myNAME ASC, rank ASC;

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END 2018-05-2510:54:26