没有记录时MySql计算返回零

Question

早上好，用这个把我的头撞在墙上，希望得到一些建议。我连接了 3 个表来显示销售数据，如下所示；

Select
  a._CommMonth As Month,
  Sum(a._CommDue) As Commission,
  Sum(a.`Network Paid`) As Company,
  Sum(a.Payable) As Adviser,
  c.Fee As Charge,
  Sum(a.Payable) - (c.Fee) As Total
From
  pms_payments.dbo_payments a 
  Inner Join pms_payments.dbo_nscontacts b 
  On b._Firm_Name = a._Firm_Name 
  Left Join pms_payments.dbo_clawback c 
  On b.ContactID = c.ContactID And a._CommMonth = c.Month
Where
  b.ContactID = 199
Group By
  a._CommMonth

Sum(a.Payable) - (c.Fee) 由于 Total 字段是我遇到问题的地方，如果 c.fee 表中不存在任何记录，则不会返回任何内容，例如 100 - [No record] = 100。如果存在记录，则按预期扣除该值，例如 100 - 15 = 85。

这有意义吗？

谢谢

Answer 1

看看使用 IFNULL

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used.

有点像

Sum(a.Payable) - IFNULL(c.Fee,0)