mysql删除重复记录，保存Id最小的一条

mysql删除重复记录，保存Id最小的一条

http://www.cnblogs.com/nzbbody/p/4470638.html

方法1：

1、创建一个临时表，选取需要的数据。

2、清空原表。

3、临时表数据导入到原表。

4、删除临时表。

mysql> select * from student;

+----+------+

| ID | NAME |

+----+------+

| 11 | aa |

| 12 | aa |

| 13 | bb |

| 14 | bb |

| 15 | bb |

| 16 | cc |

+----+------+

6 rows in set

mysql> create temporary table temp as select min(id),name from student group by name;

Query OK, 3 rows affected

Records: 3 Duplicates: 0 Warnings: 0

mysql> truncate table student;

Query OK, 0 rows affected

mysql> insert into student select * from temp;

Query OK, 3 rows affected

Records: 3 Duplicates: 0 Warnings: 0

mysql> select * from student;

+----+------+

| ID | NAME |

+----+------+

| 11 | aa |

| 13 | bb |

| 16 | cc |

+----+------+

3 rows in set

mysql> drop temporary table temp;

Query OK, 0 rows affected

这个方法，显然存在效率问题。

方法2：按name分组，把最小的id保存到临时表，删除id不在最小id集合的记录，如下：

mysql> create temporary table temp as select min(id) as MINID from student group by name;

Query OK, 3 rows affected

Records: 3 Duplicates: 0 Warnings: 0

mysql> delete from student where id not in (select minid from temp);

Query OK, 3 rows affected

mysql> select * from student;

+----+------+

| ID | NAME |

+----+------+

| 11 | aa |

| 13 | bb |

| 16 | cc |

+----+------+

3 rows in set 

方法3：直接在原表上操作，容易想到的sql语句如下：

mysql> delete from student where id not in (select min(id) from student group by name);

执行报错：1093 - You can't specify target table 'student' for update in FROM clause

原因是：更新数据时使用了查询，而查询的数据又做了更新的条件，mysql不支持这种方式。

怎么规避这个问题？

再加一层封装，如下：

mysql> delete from student where id not in (select minid from (select min(id) as minid from student group by name) b);

Query OK, 3 rows affected

mysql> select * from student;

+----+------+

| ID | NAME |

+----+------+

| 11 | aa |

| 13 | bb |

| 16 | cc |

+----+------+

3 rows in set

分类:
MySQL