database - MySQL中的加权平均计算？

Question

我目前正在使用以下查询来获取一些数字:

SELECT gid, count(gid), (SELECT cou FROM size WHERE gid = infor.gid)       
FROM infor 
WHERE id==4325 
GROUP BY gid;

我在当前阶段得到的输出如下:

+----------+-----------------+---------------------------------------------------------------+
| gid      | count(gid)      | (SELECT gid FROM size WHERE gid=infor.gid)                    |
+----------+-----------------+---------------------------------------------------------------+
|       19 |               1 |                                                            19 | 
|       27 |               4 |                                                            27 | 
|      556 |               1 |                                                           556 | 
+----------+-----------------+---------------------------------------------------------------+

我正在尝试计算加权平均值，即

(1*19+4*27+1*556)/(19+27+556)

有没有办法使用单个查询来做到这一点？

Answer 1

用途:

SELECT SUM(x.num * x.gid) / SUM(x.cou)
  FROM (SELECT i.gid,
               COUNT(i.gid) AS num,
               s.cou
          FROM infor i
     LEFT JOIN SIZE s ON s.gid = i.gid
         WHERE i.id = 4325
      GROUP BY i.gid) x