leetcode SQL Employees Earning More Than Their Managers

题目如下：

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

答案：

select e1.Name as Employee from Employee as e1,Employee as e2 where e1.Id = e2.ManagerId and e1.Salary>e2.Salary;

思路：由于这是在同一个表中对样本进行筛选，然后对不同列进行比较，因此最直观的方法是将Employee引申为两个表e1、e2，这样才能做出比较直观的筛选和比较操作。