MYSQL LEFT JOIN INCORRECT RESULT [重复]

Question

我被困了很长时间，无法找到我的查询有什么问题，LEFT JOIN 中的debit column 有问题，下面给出的Query Result image 中显示。我的debit table 的实际值为500 但MYSQL 查询显示为1500。 我在这里做错了什么。请帮帮我。

这是我的customers table

这是我的cust_credit table

这是我的cust_debit table

MYSQL Query 给出如下

SELECT 
    customers.id as id, 
    customers.cust_name AS customer_name,
    SUM(cust_debit.debit_amount) as debit,
    SUM(cust_credit.credit_amount) as credit,
    (SUM(cust_debit.debit_amount)) - (SUM(cust_credit.credit_amount)) as balance


    FROM customers
  LEFT JOIN  cust_debit  ON customers.id = cust_debit.cust_id
  LEFT JOIN  cust_credit ON customers.id = cust_credit.cust_id


  GROUP BY customers.id
  ORDER BY customers.id

我的Query Result 如下

Answer 1

信用表中有多行，这会在导致问题的组之前产生多行。如果您取出分组依据并选择所有列，您可以看到这一点。

如果您在子加入中进行分组，这个问题就会消失，那么您在分组之前每个客户都有一行。

SELECT 
  customers.id as id, 
  customers.cust_name AS customer_name,
  SUM(cust_debit.debit_amount) as debit,
  SUM(cust_credit.credit_amount) as credit,
  (SUM(cust_debit.debit_amount)) - (SUM(cust_credit.credit_amount)) as balance
FROM customers
LEFT JOIN cust_debit  ON customers.id = cust_debit.cust_id
LEFT JOIN (
  SELECT cust_id, sum(credit_amount) as credit_amount)
  from cust_credit
  group by cust_id
) cust_credit ON customers.id = cust_credit.cust_id
GROUP BY customers.id
ORDER BY customers.id