[LeetCode] Duplicate Emails 重复的邮箱

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：

巴特西