题目描述：
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。
SQL语句：
解法一：
# Write your MySQL query statement below
DELETE p1 FROM Person p1 INNER JOIN Person p2
WHERE p1.Email = p2.Email AND p1.Id > p2.Id;
上述SQL使用了MySQL DELETE语法的多表语法I，参阅：https://dev.mysql.com/doc/refman/5.0/en/delete.html
DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
tbl_name[.*] [, tbl_name[.*]] ...
FROM table_references
[WHERE where_condition]
解法二：
# Write your MySQL query statement below
DELETE FROM p1 USING Person p1 INNER JOIN Person p2
WHERE p1.Email = p2.Email AND p1.Id > p2.Id;
上述SQL使用了MySQL DELETE语法的多表语法II
DELETE [LOW_PRIORITY] [QUICK] [IGNORE]
FROM tbl_name[.*] [, tbl_name[.*]] ...
USING table_references
[WHERE where_condition]
解法三：
# Write your MySQL query statement below
DELETE FROM Person WHERE ID NOT IN
(SELECT * FROM (SELECT MIN(Id) FROM Person p GROUP BY Email) t);
注意，使用下面的SQL会抛出运行时错误：
# Write your MySQL query statement below
DELETE FROM Person WHERE ID NOT IN (SELECT MIN(Id) FROM Person GROUP BY Email);
Runtime Error Message: You can't specify target table 'Person' for update in FROM clause
Last executed input: {"headers": {"Person": ["Id", "Email"]}, "rows": {"Person": []}}
上述SQL的错误原因为：
在MySQL中，禁止在FROM子句中指定被更新的目标表。
本文链接：http://bookshadow.com/weblog/2015/03/29/leetcode-delete-duplicate-emails/
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