MySQL 分组排名 实现 ROW_NUMBER() OVER (PARTITION BY ORDER BY )

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Mysql 中因为没有ROWNUM伪列、 那么想要排名、 我们就先要搞出一列伪列、 用作排名

伪列为分组排名的核心内容

SELECT
 (@r :=@r + 1) AS rank
FROM  (SELECT @r := 0) r 
 ;

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举个例子：

创建临时表插入测试数据

CREATE TEMPORARY TABLE tm_test (num int , createdat datetime  );
INSERT INTO tm_test (num, createdat)  VALUES (1, '2018/12/28 21:41:20');
INSERT INTO tm_test (num, createdat)  VALUES (1, '2018/12/28 21:42:20');
INSERT INTO tm_test (num, createdat)  VALUES (1, '2018/12/28 21:42:20');
INSERT INTO tm_test (num, createdat)  VALUES (1, '2018/12/28 21:43:20');
INSERT INTO tm_test (num, createdat)  VALUES (1, '2018/12/28 21:44:20');
INSERT INTO tm_test (num, createdat)  VALUES (2, '2018/12/29 21:41:20');
INSERT INTO tm_test (num, createdat)  VALUES (2, '2018/12/29 21:42:20');
INSERT INTO tm_test (num, createdat)  VALUES (2, '2018/12/29 21:42:20');
INSERT INTO tm_test (num, createdat)  VALUES (2, '2018/12/29 21:43:20');
INSERT INTO tm_test (num, createdat)  VALUES (2, '2018/12/29 21:44:20');
INSERT INTO tm_test (num, createdat)  VALUES (3, '2018/12/30 21:41:20');
INSERT INTO tm_test (num, createdat)  VALUES (3, '2018/12/30 21:42:20');
INSERT INTO tm_test (num, createdat)  VALUES (3, '2018/12/30 21:42:20');
INSERT INTO tm_test (num, createdat)  VALUES (3, '2018/12/30 21:43:20');
INSERT INTO tm_test (num, createdat)  VALUES (3, '2018/12/30 21:44:20');
  SELECT * FROM tm_test;

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根据某列顺序排名：

 SELECT a.*,(@r :=@r + 1) AS rank
FROM tm_test a,(SELECT @r := 0) r 
ORDER BY createdat;

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根据某列顺序排名（相同内容排名一致）：

SELECT a.* ,
CASE 
WHEN @p=a.createdat THEN @r
WHEN @p:=a.createdat THEN @r:=@r+1
END rank
FROM tm_test a ,(SELECT @r:=0,@p:=NULL)r
ORDER BY a.createdat;

---

分组排名：

SELECT num,createdat,rank
  FROM
  (
 SELECT  a.*,IF(@p=a.num,@r:=@r+1,@r:=1) AS rank,
  @p:= a.num
  FROM tm_test a,(SELECT @p:=NULL,@r:=0)r
  ORDER BY a.num,a.createdat  
  )z;

---

分组排名（相同内容排名一致）：

 SELECT num,createdat,rank
     FROM
  (
   SELECT *,
  IF(@p=num,
      CASE 
         WHEN @s=createdat THEN @r
      WHEN @s:=createdat THEN @r:=@r+1
     END,
     @r:=1 ) AS rank,
    @p:=num,
    @s:=createdat
    FROM tm_test,(SELECT @p:=NULL,@s:=NULL,@r:=0)r
     ORDER BY num,createdat desc
     )z;