mysql 删除重复数据保留一条

验证:mysql 5.6版本

方法一:

delete a from table a left join(
select (id) from table group by studentName,classId) b on a.id=b.id where b.id is null;

方法二:

explain delete from table where id not in (select minid from (select min(id) as minid from table group by studentName,classId) b);

方法三:参考:http://www.cnblogs.com/nzbbody/p/4470638.html
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

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