182. Duplicate Emails

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.

解答：找出数据中重复的邮件，依然给出两种实现方式：

  • 直接查询，分组看个数是否大于1：
SELECT Email FROM Person GROUP BY Email HAVING COUNT(Email) > 1;
  • 联表查询：
SELECT DISTINCT p1.Email FROM Person p1,Person p2 WHERE p1.Email=p2.Email AND p1.Id!=p2.Id;