Mysql去除重复记录

torlee

转自51CTO    URL：http://dadloveu.blog.iyunv.com/715500/196309/

有关mysql删除重复记录的方法，我在网上看到很多文章，很多是照抄的，我自己按网上的方法实验了一下，没有一个sql语句就能解决的方法，不知道有没有高手可以出招。
我试验的过程如下：

mysql> select * from duplicate;
+----+-------+
| id | name  |
+----+-------+
|  1 | wang  |
|  2 | wang  |
|  3 | wdang |
|  4 | wdang |
|  5 | wdand |
|  6 | wddda |
+----+-------+
6 rows in set (0.00 sec)
select * from duplicate where id in(select min(id) from duplicate group by name);
+----+-------+
| id | name  |
+----+-------+
|  1 | wang  |
|  3 | wdang |
|  5 | wdand |
|  6 | wddda |
+----+-------+
4 rows in set (0.01 sec)
mysql> delete from duplicate where id not in(select min(id) from duplicate group by name);
ERROR 1093 (HY000): You can't specify target table 'duplicate' for update in FROM clause

最后我用了笨办法，复制无重复记录到新表格，删除旧表格，然后重命名新表格为旧表名称。

mysql> select * from duplicate where id in(select min(id) from duplicate group by name);
+----+-------+
| id | name  |
+----+-------+
|  1 | wang  |
|  3 | wdang |
|  5 | wdand |
|  6 | wddda |
+----+-------+
4 rows in set (0.01 sec)
mysql> create table duplica select * from duplicate where id in(select min(id) from duplicate group by name);
Query OK, 4 rows affected (0.02 sec)
Records: 4  Duplicates: 0  Warnings: 0
mysql> drop table duplicate;
Query OK, 0 rows affected (0.01 sec)
mysql> alter table duplica rename to duplicate;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from duplicate;
+----+-------+
| id | name  |
+----+-------+
|  1 | wang  |
|  3 | wdang |
|  5 | wdand |
|  6 | wddda |
+----+-------+
4 rows in set (0.00 sec)

mysql> alter table duplicate modify id int(2) not null primary key auto_increment;
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0


后来想了一个语句搞定了：
mysql> use mysql
Database changed
mysql> select * from duplicate;
+----+-------+
| id | name  |
+----+-------+
|  1 | wang  |
|  3 | wdang |
|  5 | wdand |
|  6 | wddda |
|  2 | wang  |
|  4 | wdang |
+----+-------+
6 rows in set (0.00 sec)
mysql> delete duplicate as a from duplicate as a,
    -> (
    -> select * from duplicate group by name having count(1)>1) as b
    -> where a.name=b.name and a.id > b.id;
Query OK, 2 rows affected (0.00 sec)
mysql> select * from duplicate;
+----+-------+
| id | name  |
+----+-------+
|  1 | wang  |
|  3 | wdang |
|  5 | wdand |
|  6 | wddda |
+----+-------+
4 rows in set (0.00 sec)