爲每個用戶在兩個表中選擇SUM

Question

我有一個包含用戶表，credit_purchase表和credit_spend表的應用程序。我正在嘗試獲取用戶當前擁有的信用額度。

信貸表如下所示：

credit_purchase

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

credit_spend

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

我試圖做的是選擇SUM（credit_purchase.coins） - SUM（ credit_spend.coins）爲每個用戶。現在我有這樣的事情：

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，它的結果是不正確的數字。這似乎是正確的購買總和，但乘以購買數量的支出乘以SUM。

我在做什麼錯？

謝謝！

Answer 1

試試這個：

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