MySQL获取分组的两个表列的总和

Question

我有3张桌子：

我想选择每个用户的总收益与总支出之差。所以我的假设表可能是：

我尝试过：

SELECT g.total - s.total AS quantity, id FROM
(SELECT SUM(quantity) AS total FROM gain GROUP BY user) AS g,
(SELECT SUM(quantity) AS total FROM spent GROUP BY user) AS s, users

但这不起作用...

Answer 1

您需要使用users表作为基本表，以便能够考虑所有用户，然后LEFT JOIN到子查询中计算总支出和总收益。这是因为某些用户在收益表或消费表中可能没有任何条目。另外，Coalesce()函数处理NULL（如果没有匹配的行）

SELECT 
  u.id AS user, 
  COALESCE(tot_gain, 0) - COALESCE(tot_spent, 0) AS balance 
FROM users AS u 
LEFT JOIN (SELECT user, SUM(quantity) as tot_spent 
           FROM spent 
           GROUP BY user) AS s ON s.user = u.id
LEFT JOIN (SELECT user, SUM(quantity) as tot_gain 
           FROM gain 
           GROUP BY user) AS g ON g.user = u.id

Answer 2

Madhur的解决方案很好。另一种选择是union all和group by：

select user, sum(gain) as gain, sum(spent) as spent
from ((select user, quantity as gain, 0 as spent
       from gain
      ) union all
      (select user, 0, quantity as spent
       from spent
      )
     ) u
group by user;

如果您希望不在表中的用户或需要其他列的用户，可以从join到user。但是，join可能不是必需的。