前言

一、题目：181. 超过经理收入的员工

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。

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。

二、解题

1.正确示范①

提交SQL

select u1.name Employee from Employee u1,Employee u2 
where u1.managerId=u2.id 
and u1.salary>u2.salary;

运行结果

2.正确示范②

提交SQL

select u1.name Employee from Employee u1
where salary>(
    select salary from Employee u2 where  u1.managerId=u2.id
);

运行结果

3.正确示范③

提交SQL

select u1.name Employee
from Employee u1
left join Employee u2 
on u1.managerId=u2.id
where u1.salary>u2.salary;

运行结果

4.正确示范④

提交SQL

select u1.name Employee
from Employee u1
join Employee u2 
on u1.managerId=u2.id
where u1.salary>u2.salary;

运行结果

5.其他

总结

正确示范①思路：
方法1会产生笛卡尔积，通过where限定符合规则的条件即可
where + and
where u1.managerId=u2.id and u1.salary>u2.salary
正确示范②思路：
采用子查询方式
where + 子查询
where salary>(select salary from Employee u2 where u1.managerId=u2.id)
正确示范③思路：
left join + on + where
注意这里的where在该题目中不能换作and
正确示范④思路：
join + on +where 或者 join + on +and