Mysql左加入总和标准

Question

我想创建一个报告，将显示客户有一个未解决的帐户余额，但我似乎无法得到它使用左连接工作。

我有3个表 - 客户，购买和支付，我想显示客户，其采购总额费用少支付大于0

到目前为止，我有以下查询：

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"); 

任何人都可以看到为什么这不工作？

感谢您的帮助！

Answer 1

您需要计算每个customerID的子查询上的每个总数，以避免不正确的结果。

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