mysql数据库查询练习二-【比经理工资还高员工姓名】【查找重复的邮箱】

1.查询工资比他经理的工资还高的员工的姓名

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

s1表：                                     s2表：
+----+-------+--------+-----------+        +----+-------+--------+-----------+
| Id | Name  | Salary | ManagerId |        | Id | Name  | Salary | ManagerId |
+----+-------+--------+-----------+        +----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |        | 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |        | 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000  | NULL      |        | 3  | Sam   | 60000  | NULL      |
| 4  | Max   | 90000  | NULL      |        | 4  | Max   | 90000  | NULL      |
+----+-------+--------+-----------+        +----+-------+--------+-----------+

2.解答

（1）方法一

select s1.Name as Employee from Employee as s1,Employee as s2 where s1.ManagerId=s2.Id and s1.Salary > s2.Salary

（2）方法二

Select s1.Name as Employee from Employee s1 join Employee s2  on s1.ManagerId = s2.Id and s1.Salary > s2.Salary

或

Select s1.Name as Employee from Employee s1 join Employee s2 on s1.ManagerId = s2.Id where s1.Salary > s2.Salary

（3）方法三:这种方法涉及到子查询，所以查询效率低

select s1.Name as Employee from Employee as s1 where s1.Salary > (select s2.Salary from Employee as s2 where s1.ManagerId = s2.Id);

3.编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

示例：

+----+---------+
| Id | Email   |
+----+---------+
| 1  | a@b.com |
| 2  | c@d.com |
| 3  | a@b.com |
+----+---------+

根据以上输入，你的查询应返回以下结果：

+---------+
| Email   |
+---------+
| a@b.com |
+---------+

说明：所有电子邮箱都是小写字母。

4.解答

• （1）方法一：

select distinct p1.Email from Person as p1,Person as p2 where p1.Email=p2.Email and p1.Id !=p2.Id;

• （2）方法二：

select Email 
	from Person 
		group by Email 
			having count(*) > 1;