此为 LeetCode 上 2021-11-07 的每日一题：【182. 查找重复的电子邮箱】

1.题目描述

编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。

示例：

Create table If Not Exists Person (id int, email varchar(255))
Truncate table Person
insert into Person (id, email) values ('1', 'a@b.com')
insert into Person (id, email) values ('2', 'c@d.com')
insert into Person (id, email) values ('3', 'a@b.com')

+----+---------+
| 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。

2.题解

【解法一】：借助HAVING语句+聚合函数

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

使用 group by 对Email分组，
然后 having 用于分组后的过滤，
其中 count(Email) 对其计数。

having 和 where的区别，where的执行在分组前，having在分组后

【解法二】：自连接

1
2
3

select DISTINCT P1.Email 
from Person p1, Person p2 
where p1.Email = p2.Email and p1.Id != p2.Id;

或

select distinct p1.email 
from Person p1 join Person p2
on p1.Email = p2.Email
where p1.Id != p2.Id;

【解法三】：子表查询

-- 先计数，再查询
SELECT 
        Email 
FROM 
        (
        SELECT Email, count(*) num 
        FROM Person 
        GROUP BY Email 
        ) t1
WHERE
        num > 1;