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