MYSQL 多表查询 LeetCode 181.超过经理收入的员工 left join on 多表查询

表：Employee 

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| name        | varchar |
| salary      | int     |
| managerId   | int     |
+-------------+---------+
Id是该表的主键。
该表的每一行都表示雇员的ID、姓名、工资和经理的ID。

编写一个SQL查询来查找收入比经理高的员工。

以 任意顺序 返回结果表。

查询结果格式如下所示。

示例 1:

输入: 
Employee 表:
+----+-------+--------+-----------+
| id | name  | salary | managerId |
+----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000  | Null      |
| 4  | Max   | 90000  | Null      |
+----+-------+--------+-----------+
输出: 
+----------+
| Employee |
+----------+
| Joe      |
+----------+
解释: Joe 是唯一挣得比经理多的雇员。

解题思路：每个员工可能既是员工也可能是经理，所以将每个员工的经理的薪资找出来。再判断员工薪资大于经理薪资。

利用left join on 进行多表查询 

select e1.name as Employee, e1.salary as EmployeeSalary ,
e2.name as manager,e2.salary as managerSalary 
from Employee e1 left join Employee e2 on e1.managerId = e2.id

在此表的基础上查询员工薪资大于经理薪资的字段

select Employee from

(select e1.name as Employee, e1.salary as EmployeeSalary ,e2.name as manager,
e2.salary as managerSalary 
from Employee e1 left join Employee e2 on e1.managerId = e2.id) e3

where EmployeeSalary > managerSalary