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：

　　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：

　　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：

　　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;