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。
Write a SQL query to find all duplicate emails in a table named Person. For example, your query should return the following for the above table~
把每封电子邮件存在的次数查询出来作为临时表，然后筛选出次数大于1的邮件即为所求；
select Email from( select Email,count(*) as num from Person group by Email ) as transient where num > 1更加简单高效的方法是group by 结合 having
select Email from Person group by Email having count(Email)>1
