【sql】统计重复出现的邮箱 Duplicate Emails

问题：

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.

解决：

① 用Group by...Having Count(*)...的固定搭配来作。987 ms

SELECT Email FROM Person GROUP BY Email
HAVING COUNT(*) > 1;

② 能够用内交来作，用Email来内交两个表，而后返回Id不一样的行，则说明两个不一样的人使用了相同的邮箱。1037 ms

SELECT DISTINCT p1.Email FROM Person p1 
JOIN Person p2 ON p1.Email = p2.Email
WHERE p1.Id <> p2.Id;

SELECT DISTINCT p1.Email FROM Person p1,Person p2
WHERE p1.Email = p2.Email AND p1.Id <> p2.Id;