Employee 表包含所有员工，他们的经理也属于员工。每个员工都有一个 Id，此外还有一列对应员工的经理的 Id。

+----+-------+--------+-----------+
| Id | Name  | Salary | ManagerId |
+----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000  | NULL      |
| 4  | Max   | 90000  | NULL      |
+----+-------+--------+-----------+

给定 Employee 表，编写一个 SQL 查询，该查询可以获取收入超过他们经理的员工的姓名。在上面的表格中，Joe 是唯一一个收入超过他的经理的员工。

+----------+
| Employee |
+----------+
| Joe      |
+----------+

答案: select * FROM employee AS a, employee AS b where a.ManagerId = b.Id and a.Salary > b.Salary
拓展:选出有经理的员工即manageredId不为空 select Id from employee where ManagerId is not null
　　:当where中id对应有多个值的时候使用in  查出不为空的经理的薪水 select ManagerId from employee where Id in (select Id from employee where ManagerId is not null)