LeetCode 181. Employees Earning More Than Their Managers

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

题目大意 :
雇员表记录了所有雇员的信息，包括他们的经理在内。每一个雇员都有一个Id，和他的经理的Id。

给定雇员表，编写一个SQL查询找出薪水大于经理的员工姓名。对于上表来说，Joe是唯一收入大于经理的员工。

解题思路

注意通过as操作将结果字段名设置为Employee

• 双表条件查询 (双表笛卡尔积)

SELECT a.NAME AS Employee FROM Employee a, Employee b 
WHERE a.ManagerId = b.Id AND a.Salary > b.Salary

• 自连接 (只返回连接条件的数据)

SELECT SELECT a.NAME FROM Employee a INNER JOIN  Employee b
ON a.ManagerId = b.Id AND a.Salary > b.Salary

两种方法速度差别不大

相关学习文档

MySQL官方文档 JOIN 规范
INNER JOIN,LEFT JION,RIGHT JION 三种表连接方法比较

以下内容摘自 : http://blog.csdn.net/scythe666/article/details/51881235

以下图两张表举例

Table A 是左边的表。Table B 是右边的表。

1.INNER JOIN 产生的结果是AB的交集

SELECT * FROM TableA INNER JOIN TableB ON TableA.name = TableB.name

INNER JOIN 产生的结果是AB的交集
INNER JOIN 产生的结果是AB的交集
2.LEFT [OUTER] JOIN 产生表A的完全集，而B表中匹配的则有值，没有匹配的则以null值取代。

SELECT * FROM TableA LEFT OUTER JOIN TableB ON TableA.name = TableB.name!

LEFT [OUTER] JOIN 产生表A的完全集，而B表中匹配的则有值

3.**RIGHT [OUTER] JOIN **产生表B的完全集，而A表中匹配的则有值，没有匹配的则以null值取代。

SELECT * FROM TableA RIGHT OUTER JOIN TableB ON TableA.name = TableB.name

图表示如left join类似。

4.FULL [OUTER] JOIN 产生A和B的并集。对于没有匹配的记录，则会以null做为值。

SELECT * FROM TableA FULL OUTER JOIN TableB ON TableA.name = TableB.name 

你可以通过is NULL将没有匹配的值找出来：

SELECT * FROM TableA FULL OUTER JOIN TableB ON TableA.name = TableB.nameWHERE TableA.id IS null OR TableB.id IS null 

FULL [OUTER] JOIN 产生A和B的并集 FULL [OUTER] JOIN 产生A和B的并集

5. CROSS JOIN 把表A和表B的数据进行一个N*M的组合，即笛卡尔积。如本例会产生4*4=16条记录，在开发过程中我们肯定是要过滤数据，所以这种很少用。