mysql - 如果第一个结果为空，如何运行第二个 SQL 查询

Question

我正在绕圈子，希望有人能阐明当第一个查询返回空值时如何运行第二个查询。

select employee, date, workingrule
case when workingrule is null
then 1 else (select employeenumber, bonusrule from employeelist)
end
from employeedetails

基本上，如果工作规则是空白的，我想找到员工奖金规则。非常感谢任何指导帮助。谢谢

Answer 1

这适用于 SQL Server（因为使用isnull，但可以将其更改为任何 RDBMS 特定查询）：

select
   ed.employee, ed.date,
   isnull(ed.workingrule, el.bonusrule) as [rule]
from employeedetails as ed
    left outer join employeelist as el on el.employeenumber = ed.employeenumber 

或者如果你想要两者（应该适用于任何 RDBMS）：

select
   ed.employee, ed.date,
   ed.workingrule,
   case when ed.workingrule is null then '1' else el.bonusrule end as bonusrule
from employeedetails as ed
    left outer join employeelist as el on el.employeenumber = ed.employeenumber 

Answer 2

select 
  employee, date, Coalesce(workingrule, BonusRule) as Rule 
from 
  employeedetails ed
  left join employeelist el on 
    el.employeenumber = ed.employeenumber