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.
这道题让我们求重复的邮箱，那么最直接的方法就是用Group by...Having Count(*)...的固定搭配来做，代码如下：
解法一：
SELECT Email FROM Person GROUP BY Email
HAVING COUNT(*) > 1;
我们还可以用内交来做，用Email来内交两个表，然后返回Id不同的行，则说明两个不同的人使用了相同的邮箱：
解法二：
SELECT DISTINCT p1.Email FROM Person p1
JOIN Person p2 ON p1.Email = p2.Email
WHERE p1.Id <> p2.Id;
参考资料：
https://leetcode.com/discuss/53206/a-solution-using-a-group-by-and-another-one-using-a-self-join