mysql 里面为什么有时候子查询需要 alias，有时候则不需要呢？

我给这个题提交了一个解答 https://leetcode.cn/problems/employees-with-missing-information/

这是我的代码：

with eid as (select employee_id
             from Employees),
     sid as (select employee_id
             from Salaries)
select employee_id
from (select e.employee_id as employee_id
       from eid e
       where e.employee_id not in
             (select * from sid)
      union
      select s.employee_id as employee_id
       from sid s
       where s.employee_id not in 
       		(select * from eid)
       ) t
order by employee_id;

可以发现，not in (select * from sid)和not in (select * from eid)这两个子查询都没有加 alias ，但可以通过，如果加了反而不通过

而

from (select e.employee_id as employee_id
       from eid e
       where e.employee_id not in
             (select * from sid)
      union
      select s.employee_id as employee_id
       from sid s
       where s.employee_id not in 
       		(select * from eid)
       ) t

这个子查询则需要加别名（我的命名是 t ）

为什么会这样呢？