Leetcode 196 Delete Duplicate Emails

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  |
+----+------------------+

Note:

Your output is the whole Person table after executing your sql. Use delete statement.

1 子查询1(报错)

先构造一个集合, 这个集合包含所有的重复邮箱的Id SELECT p1.Id FROM Person AS p1, Person AS p2 WHERE p1.Email = p2.Email AND p1.Id > p2.Id

然后再删除在这个集合中的邮箱.

# error You cant specify target table 'p' for update in FROM clause
DELETE FROM Person WHERE Id IN 
(
    SELECT p1.Id FROM Person AS p1, Person AS p2 WHERE p1.Email = p2.Email AND p1.Id > p2.Id
);

错误的原因是子查询和删除操作不能使用同一张表!

2 子查询2

解决子查询1的做法是, 把子查询的结果重命名一下, 重命名到Tmp上, 这样就不会产生问题了

# Write your MySQL query statement below
DELETE FROM Person WHERE Id IN 
(
    SELECT Id FROM (SELECT p1.Id FROM Person AS p1, Person AS p2 WHERE p1.Email = p2.Email AND p1.Id >
                    p2.Id) AS Tmp
);