【sql学习】LeetCode之196. Delete Duplicate Emails_sql write a sql query to delete all duplicate emai

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

 

别人的代码：

Approach: Using DELETE and WHERE clause [Accepted]

Algorithm

By joining this table with itself on the Email column, we can get the following code.

SELECT p1.*
FROM Person p1,
    Person p2
WHERE
    p1.Email = p2.Email
;

Then we need to find the bigger id having same email address with other records. So we can add a new condition to the WHERE clause like this.

SELECT p1.*
FROM Person p1,
    Person p2
WHERE
    p1.Email = p2.Email AND p1.Id > p2.Id
;

As we already get the records to be deleted, we can alter this statement to DELETE in the end.

MySQL

DELETE p1 FROM Person p1,
    Person p2
WHERE
    p1.Email = p2.Email AND p1.Id > p2.Id

解释：

https://leetcode.com/problems/delete-duplicate-emails/discuss/55553/Simple-Solution 

 

方法二：使用中间表

delete from Person where id not in(select min(id) as id from Person group by email)

you will be noted " You can't specify target table 'Person' for update in FROM clause ",
The solution is using a middle table with select clause:

delete from Person where id not in( 
    select t.id from (
        select min(id) as id from Person group by email
    ) t
)

小结：

1）min()的用法

2）使用中间表避免“You can't specify target table 'Person' for update in FROM clause”

      因为mysql中，不能先select一个表的记录，再按此条件进行更新和删除同一个表的记录。

      解决办法是，将select得到的结果，再通过中间表select一遍，这样就规避了错误，这个问题只出现于mysql，mssql和oracle不会出现此问题。

3）group by的用法