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 |
+----------+
这道题给我们了一个Employee表，里面有员工的薪水信息和其经理的信息，经理也属于员工，其经理Id为空，让我们找出薪水比其经理高的员工，那么就是一个很简单的比较问题了，我们可以生成两个实例对象进行内交通过ManagerId和Id，然后限制条件是一个Salary大于另一个即可：
解法一：
SELECT e1.Name FROM Employee e1
JOIN Employee e2 ON e1.ManagerId = e2.Id
WHERE e1.Salary > e2.Salary;
我们也可以不用Join，直接把条件都写到where里也行：
解法二：
SELECT e1.Name FROM Employee e1, Employee e2
WHERE e1.ManagerId = e2.Id AND e1.Salary > e2.Salary;
参考资料：
https://leetcode.com/discuss/88189/two-straightforward-way-using-where-and-join