LeetCode：196. Delete Duplicate Emails

题目：
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 |
+—-+——————+

Answer：
思路：使用DELETE FROM，注意用MAX找出最大的id不行，因为有多组重复的情况。

# Write your MySQL query statement below
DELETE A1 
FROM
    person AS A1,
    person AS A2 
WHERE
    A1.Email = A2.Email 
    AND A1.Id > A2.Id