MySQL表的多列的SUM

Question

这是tbl_user：

-----------------
ID    | Username
-----------------
1       user one
2       user two

这是tbl_x1：

-------------------------
ID    | User_id | Sum
-------------------------
1       1         10
2       1         20
3       2         30

这是tbl_x2：

-------------------------
ID    | User_id | Sum
-------------------------
1       1         10
2       1         20
3       1         30

这是tbl_y：

-------------------------
ID    | User_id | Days
-------------------------
1       1         10

我想选择sum（x1.sum + x2.sum）和sum（y.Days） 换句话说，我想要以下结果：

-------------------------------------
ID    |  Username  |  Sum  |  Days
-------------------------------------
1       user one        90     10
2       user two        30     0

我尝试使用此代码：

select u.id as ID, u.username as Username, sum(y.days) as Days,(ifnull(sum( x1.sum), 0) + ifnull(sum( x2.sum), 0)) as Sum from tbl_user u left join tbl_x1 x1 on u.id = x1.user_id left join tbl_x2 g on u.id = x2.user_id left join tbl_y y on u.id = y.user_id group by u.id

但是我得到了错误的结果。

Answer 1

问题是join操作会乘以给定user_id的行。通过乘以行（称为“笛卡尔积”），您得到的是错误的总和。

解决方案是在进行连接之前进行聚合：

select u.id as ID, u.username as Username, y.days,
       coalesce(x1.sum, 0) + coalesce(x2.sum, 0)) as Sum
from tbl_user u left join
     (select x1.user_id, sum(x1.sum) as sum
      from tbl_x1 x1
      group by x1.user_id
     ) x1
     on u.id = x1.user_id left join
     (select x2.user_id, sum(x.sum) as sum
      from tbl_x2 x2
      group by x2.user_id
     ) x2
     on u.id = x2.user_id left join
     (select y.user_id, sum(y2.days) as days
      from tbl_y y
      group by y.user_id
     ) y
     on u.id = y.user_id;