README.md

Employees Earning More Than Their Managers

Problem

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

Solution 1: 相关子查询

select e1.Name as Employee from Employee2 as e1
where e1.ManagerId is not NULL
and e1.Salary > (select Salary from Employee2 where Id=e1.ManagerId);

结果超时，原因是相关子查询效率非常低，应该尽量避免

Solution 2: 内连接

select e1.Name as Employee from Employee2 as e1
join Employee2 as e2
on e1.ManagerId = e2.Id and e1.Salary > e2.Salary
where e1.ManagerId is not NULL;

结果顺利通过。