代码在mysql中加倍netsalary

Question

请有人告诉我为什么以下代码不会从扣除表中扣除金额，然后从出勤表中的金额中扣除以获得净工资。在应用扣除之前，它不是总结和扣除，而是将出勤金额的总和加倍，我非常感激。

select sum(attendance.amount) - max(deduction.amount) 
from attendance 
join deduction on attendance.staffid = deduction.staffid 
where attendance.staffid = some_staffid 
    and month(attendance.date) = some_month
    and month(deduction.date_approved) = some_month

Answer 1

只是一个疯狂的猜测 - 每次出勤都有2个扣除记录=＆gt;联接将复制出勤表中的金额=＆gt;总和将对这些重复的记录进行求和

简单的解决方案（但表现不是很好，我很害怕）将是：

select sum(a) - max_d
from (
  select attendance.amount a, deduction.amount d, max(deduction.amount) max_d
  from attendance 
  join deduction on attendance.staffid = deduction.staffid 
  where attendance.staffid = some_staffid 
      and month(attendance.date) = some_month
      and month(deduction.date_approved) = some_month
)
where d = max_d