The Employee table holds all employees including their managers. Every employee has an Id, and there is also a column for the manager Id.

+----+-------+--------+-----------+| Id | Name  | Salary | ManagerId |+----+-------+--------+-----------+| 1  | Joe   | 70000  | 3         || 2  | Henry | 80000  | 4         || 3  | Sam   | 60000  | NULL      || 4  | Max   | 90000  | NULL      |+----+-------+--------+-----------+
Given the Employee table, write a SQL query that finds out employees who earn more than their managers. For the above table, Joe is the only employee who earns more than his manager.
+----------+| Employee |+----------+| Joe      |+----------+
分析：
首先找到哪些是雇員，即ManagerId is not NULL;
下面是找到哪些雇員的工資比上級高，因為都是在同一個表中，所以應該給表起個別名。重點在Id=a,ManagerId;
select Name as Employee from Employee awhere ManagerId is not NULLand Salary > (select Salary from Employee where Id=a.ManagerId);