mysql - 代码在mysql中加倍netsalary

Question

请谁能告诉我为什么下面的代码不从扣除表中计算金额，然后从出勤表中的金额总和中扣除以获得净工资。在应用扣除之前，它不会将出勤表中的金额总和加倍，而不是相加和扣除，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 - 每个出勤都有 2 条扣除记录 => 连接将复制出勤表中的金额 => 总和将汇总这些重复的记录

简单的解决方案（但我害怕表现不佳）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