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 | +---------+方法一： 看到“查重复”这样的题，首先要用分组函数（group by），再用聚合函数count给email计数直接做判断
select Email From Person group by Email having count(Email)>=2方法二： 还有一个使用临时表的方式，稍稍有点麻烦
select Email, count(Email) as num from Person group by Email将上表作为临时表，查出不同的email有的个数，然后再判断num>=2所对应的Email 具体sql语句如下：
select Email from ( select Email, count(Email) as num from Person group by Email ) as statistic where num >= 1