Write a SQL query to find all duplicate emails in a table named Person.

    +----+---------+
    | Id | Email   |
    +----+---------+
    | 1  | a@b.com |
    | 2  | c@d.com |
    | 3  | a@b.com |
    +----+---------+
    For example, your query should return the following for the above table:

    +---------+
    | Email   |
    +---------+
    | a@b.com |
    +---------+
Note: All emails are in lowercase.
    主要是group by的应用
    select Email from ( select Email,count(Email) as cnt from Person group by Email ) t where cnt>1;
    或者
    select Email from ( select Email,count(Email) as cnt from Person group by Email having cnt>1) t ;
    order by 要在group by 之后使用，不能在group by之前使用。
    select Email from ( select Email,count(Email) as cnt from Person order by Email group by Email ) t where cnt>1;
    这个执行会有问题。