196. Delete Duplicate Emails

problem

Write a SQL query to delete all duplicate email entries in a table named Person,
keeping only unique emails based on its smallest Id.

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
Id is the primary key column for this table.
For example, after running your query, the above Person table should have the
following rows:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+


题目大意：
编写SQL删除Person表中所有的重复email条目，只保留Id最小的唯一email记录。

其中，Id是表的主键。

解答

1. 链接自身寻找找重复

select * from Person p1, Person p2  where p1.Email = p2.Email 

2. 每个重复筛选出1个

select * from Person p1, Person p2  where p1.Email = p2.Email  and p1.id > p2.id

3. 删除重复

delete p1 from Person p1, Person p2 where p1.Email = p2.Email and p1.Id > p2.Id