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

这种单表比较条件，一般都是表内进行join操作.
参照此思路，解题如下所示：

# Write your MySQL query statement below
SELECT 
    a.Name AS Employee 
FROM Employee a, Employee b
WHERE
    a.ManagerId = b.Id
    AND a.Salary > b.Salary; 

运行效率在可以接受的范围，此外语句也较为清晰便于维护.