leetcode: Employees Earning More Than Their Managers

原题如下：

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

本题有两种思路，一种是表的自身连接不使用join关键字，另一种是使用join关键字，第二种效率比较高。

第一种代码如下，

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

表名为Employee，e1和e2分别是为Employee起的别名。

第二种代码如下，

SELECT employer.Name
FROM  Employee employer JOIN Employee manager ON (employer.ManagerId = manager.Id )
WHERE employer.Salary > manager.Salary ;

其中employer和manager也是Employee的别名，与JOIN一起使用的ON表示join的字段。

Mysql还有一个关键字left join，表示不管第二个表中有没有数据都把第一个表中的数据打印出来，leetcode的另一个题目考察了一个用法

题目连接：https://leetcode.com/problems/combine-two-tables/

解答：

select FirstName,LastName,City,State
from Person left join Address on Person.PersonId=Address.PersonId因为不存在重复，所以在select可以不用使用表名作为前缀