mysql - MySQL 获取 ORDER BY 编号错误的行位置

Question

我有以下 mySQL 查询，我想给在计算字段上排序的行以及相应的行位置。我已使用此处的 OMG Ponies 代码作为模板，但返回的行编号不正确（它们在没有排序的情况下按键（gemid）顺序编号）。我知道这与 GROUP BY 上的 JOIN 有关，但我知道如何解决它。提前致谢。

SELECT g.gemid, sum_rating, @rownum := @rownum + 1 AS row_num FROM gems g
    LEFT JOIN (SELECT gemid, SUM(rating) as sum_rating from gemrating GROUP BY gemid) rt ON g.gemid = rt.gemid 
    JOIN (SELECT @rownum := 0) Z 
    WHERE g.grade = '8'  
    ORDER BY sum_rating asc

输出应该看起来像：

gemid    sum_rating    row_num
------   ------------  ----------
2           10           1
4           25           2
1           40           3
3           41           4

相反，它又回来了：

gemid    sum_rating    row_num
------   ------------  ----------
2           10           2
4           25           4
1           40           1
3           41           3

Answer 1

看起来这行得通。我知道在编号之前必须先对记录进行排序，然后从有序列表中选择。

SELECT g2.gemid, g2.sum_rating, @rownum := @rownum + 1 AS row_num FROM 

(SELECT g.gemid, rt.sum_rating, g.grade FROM gems g
    LEFT JOIN (SELECT gemid, SUM(rating) as sum_rating from gemrating GROUP BY gemid) rt ON g.gemid = rt.gemid 
 WHERE g.grade = '8'  ) g2

    JOIN (SELECT @rownum := 0) Z 
    WHERE g2.grade = '8'  
    ORDER BY sum_rating asc