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      |
+----------+

解题：
1.复制表，分别为t1,t2
2.找出两表中id 与 managerid相同，并且雇员工资更高的名字
3. 将name 改为 Employee

select  t1.name  as ‘Employee‘  from

　　Employee as t1,

　　(select * from Employee) as t2 

　　where

　　　　t1.managerid = t2.id

　　and

　　　　t1.salary > t2.salary

此处需要注意的是：manager 应该是id的直系manager，起初我的思路是找到 ManagerId  is null 的数据，放到 表 t2中，然后继续按照步骤比较两个表中数据，但这就会出现雇员1 可能有 manager1，但是同时manager1 也会有manager2 作为他的manager，这就会导致我表t2中的manager数据有遗漏。导致结果错误。

错误写法：

select  t1.name  as ‘Employee‘  from

　　Employee as t1,

　　(select * from Employee where managerid is null) as t2 

　　where

　　　　t1.managerid = t2.id

　　and

　　　　t1.salary > t2.salary