mysql INNER JOIN 上的值错误

Question

我得到的 gfee 和 netpay 值错误。

SELECT s.id, s.name, c.name AS course_name,
s.open_bal AS open_balance, sum(i.amount) AS gross_fee,
sum(i.discount) AS discount, sum(i.amount) - sum(i.discount) AS net_payable,
SUM(r.reg_fee+r.tut_fee+r.other_fee) AS net_recieved,
(sum(i.amount) - sum(i.discount)) - SUM(r.reg_fee+r.tut_fee+r.other_fee) AS balance_due
FROM subscribers s
INNER JOIN courses c on c.id = s.course_id
LEFT JOIN invoices i on i.student_id = s.id
LEFT JOIN recipts r on r.student_id = s.id
GROUP BY s.id;

为什么会发生这种情况？

Answer 1

70,000 是 35,000 的两倍这一事实是一个非常重要的线索。您正在谈论 SUM 并且您得到的结果是您预期的两倍。这强烈表明您加入的行数是您想象的两倍。

如果不了解架构的详细信息，则很难具体说明，但如果您要为 invoices 中的每一行（对于此特定记录）连接到 receipts 中的两个不同行，您最终会在总和中包含两个 35,​​000 条目。不是你想的那样。

Answer 2

SELECT s.id
     , s.name
     , c.name AS course_name
     , s.open_bal AS open_balance
     , igroup.gross_fee
     , igroup.discount
     , igroup.net_payableinvoices
     , rgroup.net_recieved
     , igroup.net_payableinvoices - rgroup.net_recieved
       AS balance_due
FROM students s
INNER JOIN courses c
  on c.id = s.course_id
LEFT JOIN 
  ( SELECT i.student_id
         , SUM(i.amount) AS gross_fee
         , SUM(i.discount) AS discount
         , SUM(i.amount) - sum(i.discount)
           AS net_payableinvoices
    FROM invoices i
    GROUP BY i.student_id
  ) AS igroup
  ON igroup.student_id = s.id
LEFT JOIN 
  ( SELECT r.student_id
         , SUM(r.reg_fee+r.tut_fee+r.other_fee)
           AS net_recieved
    FROM recipts r 
    GROUP BY r.student_id
  ) AS rgroup
  ON rgroup.student_id = s.id
;

Answer 3

问题最可能的原因是连接表之一中存在多行。

由于您获得的值恰好是预期值的两倍（70000 与 35000），我猜测在 courses 或 recipts 表中有两行带有 student_id=22。