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Duplicate Emails

栏目:数据库应用时间:2015-05-12 09:03:46
    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的利用
    select Email from ( select Email,count(Email) as cnt from Person group by Email ) t where cnt>1;
    或
    select Email from ( select Email,count(Email) as cnt from Person group by Email having cnt>1) t ;
    order by 要在group by 以后使用,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;
    这个履行会有问题。
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