LeetCode——Delete Duplicate Emails（巧用mysql临时表）

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  | [e-mail] |
| 2  | [e-mail]  |
| 3  | [e-mail] |
+----+------------------+
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  | [e-mail] |
| 2  | [e-mail]  |
+----+------------------+
note:

your output is the whole person table after executing your sql. use delete statement.

此题有两个解法：

我初步尝试用以下sql解决问题(要删除的记录id肯定大于相同内容的id):

delete p1 from person p1, person p2 where p2.id > p1.id and p2.email = p1.email;

但是无法通过，究其原因是在sql语句中，select与delete操作不能同时存在.

答案一

因此尝试新的解法，直接使用删除语句,结果如下所示:

delete p1 from person p1, person p2 where p1.id > p2.id and p1.email = p2.email;

此答案通过测试，但是效率较低.

答案二

后续思考中发现，可以使用临时表解决select与delete同时存在的问题，答案如下所示:

delete from person
where id in
(
    select id from
        (select p1.id as id from person p1, person p2 where p1.email = p2.email and p1.id > p2.id) as temp
)

此解决方案完美解决问题，且sql语句比较清晰明了.