LeetCode:181. Employees Earning More Than Their Managers

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

firstly，Select *  From Employee a and Employee b，get the result as below

Noticed that in the last three columns the Id is equal to Joe's, Henery's id Sam's Id  as described in this question we need to find a.manager id  is equal to b.id，when meet this requirement we need to make our a.salary is larger than b.salary.

mysql> SELECT * FROM EMPLOYEE a,EMPLOYEE b;
+----+--------+--------+-----------+----+--------+--------+-----------+
| Id | Name   | Salary | ManagerId | Id | Name   | Salary | ManagerId |
+----+--------+--------+-----------+----+--------+--------+-----------+
|  1 | Joe    | 70000  |         3 |  1 | Joe    | 70000  |         3 |
|  2 | Henery | 80000  |         4 |  1 | Joe    | 70000  |         3 |
|  3 | Sam    | 60000  |      NULL |  1 | Joe    | 70000  |         3 |
|  4 | Max    | 90000  |      NULL |  1 | Joe    | 70000  |         3 |
|  1 | Joe    | 70000  |         3 |  2 | Henery | 80000  |         4 |
|  2 | Henery | 80000  |         4 |  2 | Henery | 80000  |         4 |
|  3 | Sam    | 60000  |      NULL |  2 | Henery | 80000  |         4 |
|  4 | Max    | 90000  |      NULL |  2 | Henery | 80000  |         4 |
|  1 | Joe    | 70000  |         3 |  3 | Sam    | 60000  |      NULL |
|  2 | Henery | 80000  |         4 |  3 | Sam    | 60000  |      NULL |
|  3 | Sam    | 60000  |      NULL |  3 | Sam    | 60000  |      NULL |
|  4 | Max    | 90000  |      NULL |  3 | Sam    | 60000  |      NULL |
|  1 | Joe    | 70000  |         3 |  4 | Max    | 90000  |      NULL |
|  2 | Henery | 80000  |         4 |  4 | Max    | 90000  |      NULL |
|  3 | Sam    | 60000  |      NULL |  4 | Max    | 90000  |      NULL |
|  4 | Max    | 90000  |      NULL |  4 | Max    | 90000  |      NULL |
+----+--------+--------+-----------+----+--------+--------+-----------+

the result is 

SELECT * From Employee a,Employee b

WHERE a.managerId = b.Id AND a.Salary>b.Salary