Mysql5.7版本实现窗口函数

SELECT a.sid,a.score,
-- a.cid , 不能有该字段，添加该字段，不能分组排序，是全局排序
( @rank := CASE WHEN @cid = a.cid THEN @rank + 1 ELSE 1 END ) AS row_num,    
( @cid := a.cid ) AS cid2    
-- 字段中必须有字段逻辑 ( @cid := a.cid )，否则不能实现分组排序；且分组字段必须在排序字段后面，否则是针对所有数据排序
FROM ( select *
        From
        (
        -- sid ：学升；cid ：课程编码； score ：成绩
        select '小明' sid,'cid-01' cid, 90 score union all 
        select '小明' sid,'cid-02' cid, 67 score union all
        select '小明' sid,'cid-03' cid, 88 score union all
        select '张三' sid,'cid-01' cid, 78 score union all
        select '张三' sid,'cid-02' cid, 66 score union all
        select '张三' sid,'cid-03' cid, 55 score union all
        select '李四' sid,'cid-01' cid, 99 score union all
        select '李四' sid,'cid-02' cid, 94 score union all
        select '李四' sid,'cid-03' cid, 90 score 
        )t
        ORDER BY cid, sid DESC
        ) as a
INNER JOIN ( SELECT @rank := 0, @cid := 0 ) AS b ON 1 =1;

执行结果：