[SQL] LeetCode 196. 删除重复的电子邮箱 - [Easy] __Mark

196. 删除重复的电子邮箱

题目：

编写一个 SQL 查询，来删除 Person 表中所有重复的电子邮箱，重复的邮箱里只保留 Id 最小的那个。

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
| 3  | [email protected] |
+----+------------------+

Id 是这个表的主键。

例如，在运行你的查询语句之后，上面的 Person 表应返回以下几行:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
+----+------------------+

---

解题语句：

方法一：NOT IN + 子查询

DELETE FROM
	Person 
WHERE
	Id NOT IN ( SELECT p.minId FROM ( SELECT MIN( Id ) AS minId FROM Person GROUP BY Email ) AS p );

方法二：INNER JOIN （较慢）

DELETE
	p1.* 
FROM
	Person p1
	INNER JOIN Person p2 ON p1.Email = p2.Email 
	AND p1.id > p2.Id;

---

注意：

方法一中的语句不能写为：（下面错误写法）

-- 错误写法
DELETE FROM
	Person 
WHERE
	Id NOT IN ( SELECT MIN( Id ) AS minId FROM Person GROUP BY Email );
	

报错信息：

ERROR 1093 (HY000): You can't specify target table 'Person' for update in FROM clause

MySQL官方文档的解释为：

详见：13.2.11.10 Subquery Errors
详见：C.4 Restrictions on Subqueries
解释：可以在UPDATE语句中使用子查询进行赋值， 因为子查询在语句 UPDATE 和 DELETE 语句以及 SELECT 语句中都是合法的。但是，不能使用一张相同的表同时放在子查询的 FROM 子句和修改目标中 。

因此，可以修正为：

DELETE FROM
	Person 
WHERE
	Id NOT IN ( SELECT * FROM ( SELECT MIN( Id ) AS minId FROM Person GROUP BY Email ) AS p );