181. Employees Earning More Than Their Managers--自连接

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题目：

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

这个解决方法好像是叫自连接？

select E1.Name from Employee as E1, Employee as E2 where E1.ManagerId = E2.Id and E1.Salary > E2.Salary

评论里说内连接的速度可以快一点

select a.Namefrom Employee a inner join Employee b on a.ManagerId=b.Idwhere a.Salary>b.Salary

还有一位大哥说，因为不需要没有经理的员工所以可以这样

SELECT e1.`Name`FROM `Employee` e1 LEFT JOIN `Employee` e2 ON (e2.`Id` = e1.`ManagerId`)WHERE e1.`ManagerId` IS NOT NULL AND e1.`Salary` > e2.`Salary`

事实上inner join已经做到这一点了
INNER JOIN 关键字在表中存在至少一个匹配时返回行。

拓展阅读：

sql内连接、外连接和自连接查询

http://blog.csdn.net/zdp072/article/details/42458569

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