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。