leetcode【sql】 Delete Duplicate Emails

原文链接： http://www.cnblogs.com/LUO77/p/4950243.html

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  | [email protected] |
| 2  | [email protected]  |
| 3  | [email protected] |
+----+------------------+

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  | [email protected] |
| 2  | [email protected]  |
+----+------------------+

DELETE p1 FROM Person p1 INNER JOIN Person p2 WHERE p1.Email = p2.Email AND p1.Id > p2.Id;

这里p1和p2都是Person的别名。inner join等于join。所以p1 join p2应该是以下的结果：

------+------+-----------------+
|P1.Id| p2.Id|Email            |
+-----+------+-----------------+
| 1  |   1  | [email protected] |
| 2  |   2  | [email protected]  |
| 3  |   3  | [email protected] |
+----+-------------------------+
然后p1.id>p2.id所以id为3的被删除了，但是因为p1和p2都是person事实上，只有一个id。为了理解，写成了两列。

---

补充一些关于sql语句的知识：

• sql表别名的用法：as

　　select * from kettleoutputtable a
　　where a.os =2 and storename = 'anzhi'

　　和

　　select * from kettleoutputtable as a
　　where a.os =2 and storename = 'anzhi'

　　等效，也就是说别名的as可以省略！在表明后直接加上简单的名字就行了。

转载于:https://www.cnblogs.com/LUO77/p/4950243.html