Mysql左加入總和標準

Question

我想創建一個報告，將顯示客戶有一個未解決的帳戶餘額，但我似乎無法得到它使用左連接工作。

我有3個表 - 客戶，購買和支付，我想顯示客戶，其採購總額費用少支付大於0

到目前爲止，我有以下查詢：

mysqli_query("SELECT customers.name as name, SUM(purchases.cost) as cost, SUM(payments.paymentamount) as payments FROM customers 
LEFT JOIN purchases ON purchases.customerid = customers.id 
LEFT JOIN payments ON payments.customerid = customers.id 
WHERE (cost - payments) > 0 
GROUP BY customers.id"); 

任何人都可以看到爲什麼這不工作？

感謝您的幫助！

Answer 1

您需要計算每個customerID的子查詢上的每個總數，以避免不正確的結果。

SELECT a.*, 
     COALESCE(b.totalPurchase , 0) - COALESCE(c.totalPayment , 0) AS Balance 
FROM customers a 
     LEFT JOIN 
     (
      SELECT customerid, SUM(cost) totalPurchase 
      FROM purchases 
      GROUP BY customerid 
     ) b ON a.ID = b.customerID 
     LEFT JOIN 
     (
      SELECT customerid, SUM(paymentamount) totalPayment 
      FROM payments 
      GROUP BY customerid 
     ) c ON a.ID = c.customerID 
WHERE COALESCE(b.totalPurchase , 0) - COALESCE(c.totalPayment , 0) > 0