[SQL] LeetCode 181. 超过经理收入的员工 - [Easy] __Mark

181. 超过经理收入的员工

题目：

Employee 表包含所有员工，他们的经理也属于员工。每个员工都有一个 Id，此外还有一列对应员工的经理的 Id。

+----+-------+--------+-----------+
| Id | Name  | Salary | ManagerId |
+----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000  | NULL      |
| 4  | Max   | 90000  | NULL      |
+----+-------+--------+-----------+

给定 Employee 表，编写一个 SQL 查询，该查询可以获取收入超过他们经理的员工的姓名。

在上面的表格中，Joe 是唯一一个收入超过他的经理的员工。

+----------+
| Employee |
+----------+
| Joe      |
+----------+

---

SQL架构：

Create table If Not Exists Employee (Id int, Name varchar(255), Salary int, ManagerId int);
Truncate table Employee;
insert into Employee (Id, Name, Salary, ManagerId) values ('1', 'Joe', '70000', '3');
insert into Employee (Id, Name, Salary, ManagerId) values ('2', 'Henry', '80000', '4');
insert into Employee (Id, Name, Salary, ManagerId) values ('3', 'Sam', '60000', Null);
insert into Employee (Id, Name, Salary, ManagerId) values ('4', 'Max', '90000', Null);

---

解题语句：

方法：多表查询

SELECT
	e.Name AS Employee 
FROM
	Employee AS e
	LEFT JOIN ( SELECT * FROM Employee GROUP BY Id ) AS m ON e.ManagerId = m.Id 
WHERE
	e.Salary > m.Salary;

• 注：对 id 去重，排除重复数据，减少连接的数据源，提高效率

也可以进一步写为：

SELECT
	e.Name AS Employee 
FROM
	( SELECT * FROM Employee WHERE ManagerId IS NOT NULL ) AS e
	LEFT JOIN ( SELECT * FROM Employee GROUP BY Id ) AS m ON e.ManagerId = m.Id 
WHERE
	e.Salary > m.Salary;

• 注：使用LEFT JOIN时，如果有多个条件，要注意ON和WHERE的使用区别，会影响多表查询结果。查看具体分析