SQL.182.(简单)查找重复的电子邮箱

题目：
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。

解法1:

select email 
from person 
group by email 
having count(email)>1

解法2:

select email 
from 
(select count(1) as cnt,email 
from person 
group by email) r  
where r.cnt>1;

解法3:

select distinct a.Email 
from Person a, Person b 
where a.Email = b.Email 
and a.Id != b.Id