我目前有两个版本的sql query，我正在努力了解哪个版本在性能、成本等方面更好。
这是关于这个问题：平均工资：部门vs公司

select department_salary.pay_month, department_id,
case
  when department_avg>company_avg then 'higher'
  when department_avg<company_avg then 'lower'
  else 'same'
end as comparison
from
(
  select department_id,
         avg(amount) as department_avg,
         date_format(pay_date, '%Y-%m') as pay_month
      from salary
      join employee on salary.employee_id = employee.employee_id
      group by department_id, pay_month
) as department_salary
join
(
  select avg(amount) as company_avg,
         date_format(pay_date, '%Y-%m') as pay_month
      from salary
      group by date_format(pay_date, '%Y-%m')
) as company_salary
on department_salary.pay_month = company_salary.pay_month
;

我的问题不是加入，而是在case语句中使用第二个表。

select a.pay_month,
       a.department_id,
       (case when avg(a.salary) > (
             select avg(e.salary)
                 from employee e
                 where e.pay_month = a.pay_month)
        then 'higher'
        else 'lower' end) as comparison
from employee a
group by a.pay_month,a.department_id;

哪个更好？加入真的有必要吗？