代码中的(select employee_id from Employees)为什么不能写成Employees.employee_id

Question

Write your MySQL query statement below

select employee_id from (select *
from Employees
where manager_id is not null ) AS a
where a.manager_id not in (select employee_id from Employees)
order by employee_id desc

代码中的(select employee_id from Employees)为什么不能写成Employees.employee_id

Answer 1

这涉及到了执行顺序的问题，sql语句的执行是先执行from，即先要有表，你的form后面没有employees表，你怎么在where中使用呢？你from后面是把 (select * from Employees where manager_id is not null )这个查询结果作为表的，并不是employees表。