为每个用户在两个表中选择SUM

Question

我有一个包含用户表，credit_purchase表和credit_spend表的应用程序。我正在尝试获取用户当前拥有的信用额度。

信贷表如下所示：

credit_purchase

+----+---------+-------+---------------------+ 
| id | user_id | coins | timestamp   | 
+----+---------+-------+---------------------+ 

credit_spend

+----+---------+-------+---------------------+ 
| id | user_id | coins | timestamp   | 
+----+---------+-------+---------------------+ 

我试图做的是选择SUM（credit_purchase.coins） - SUM（ credit_spend.coins）为每个用户。现在我有这样的事情：

SELECT users.*, COALESCE(SUM(credit_purchase.coins) - SUM(credit_spend.coins), 0) 
FROM users 
LEFT JOIN credit_purchase ON users.id = credit_purchase.user_id 
LEFT JOIN credit_spend ON users.id = credit_spend.user_id 
GROUP BY users.id 

但是，它的结果是不正确的数字。这似乎是正确的购买总和，但乘以购买数量的支出乘以SUM。

我在做什么错？

谢谢！

Answer 1

试试这个：

SELECT users.*, 
     COALESCE(SUM(credit_purchase.coins) - SUM(credit_spend.coins), 0) 
FROM users 
      LEFT JOIN credit_purchase 
        ON users.id = credit_purchase.user_id 
      LEFT JOIN credit_spend 
        ON users.id = credit_spend.user_id 
GROUP BY users.id