mysql中删除重复记录sql语句

删除重复记录方法一：

1. 新建一个临时表

代码如下	复制代码
create table tmp as select * from youtable group by name（name为不希望有重复的列）

2. 删除原来的表

代码如下	复制代码
drop table youtable

3. 重命名表

代码如下	复制代码
alter table tmp rename youtable

但是这个方法有个问题，由临时表转变过来的最终表，其表结构会和原来的不一致，需要手工更改。这个问题，待解决。
删除重复记录方法二：

1. 新建一个临时表

代码如下	复制代码
CREATE TABLE tmp AS SELECT * FROM youtable GROUP BY name（name为不希望有重复的列）

2. 清空原来的表

代码如下	复制代码
TRUNCATE TABLE youtable

3. 把临时表插入到youtable

代码如下	复制代码
INSERT INTO tablename SELECT * FROM temp

4. 删除临时表

代码如下	复制代码
DROP TABLE temp

删除重复记录方法三：

代码如下	复制代码
delete table where ID not in(select min(ID) from table group by name(name:重复的字段))

删除重复记录方法四：

具体实现如下：

代码如下

复制代码

Table         Create Table
------------ --------------------------------------------------------
users_groups CREATE TABLE `users_groups` (
                `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
                `uid` int(11) NOT NULL,
                `gid` int(11) NOT NULL,
                PRIMARY KEY (`id`)
              ) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8

users_groups.txt内容：

1,11,502
2,107,502
3,100,503
4,110,501
5,112,501
6,104,502
7,100,502
8,100,501
9,102,501
10,104,502
11,100,502
12,100,501
13,102,501
14,110,501

mysql> load data infile 'c:\users_groups.txt' into table users_groups fields
terminated by ',' lines terminated by 'n';
Query OK, 14 rows affected (0.05 sec)
Records: 14 Deleted: 0 Skipped: 0 Warnings: 0

mysql> select * from users_groups;

query result(14 records)

id uid gid
1 11 502
2 107 502
3 100 503
4 110 501
5 112 501
6 104 502
7 100 502
8 100 501
9 102 501
10 104 502
11 100 502
12 100 501
13 102 501
14 110 501
14 rows in set (0.00 sec)

根据一位兄弟的建议修改。

代码如下

复制代码

mysql> create temporary table tmp_wrap select * from users_groups group by uid having count(1) >= 1;
Query OK, 7 rows affected (0.11 sec)
Records: 7 Duplicates: 0 Warnings: 0

mysql> truncate table users_groups;
Query OK, 14 rows affected (0.03 sec)

mysql> insert into users_groups select * from tmp_wrap;
Query OK, 7 rows affected (0.03 sec)
Records: 7 Duplicates: 0 Warnings: 0

mysql> select * from users_groups;

query result(7 records)
id uid gid
1 11 502
2 107 502
3 100 503
4 110 501
5 112 501
6 104 502
9 102 501

mysql> drop table tmp_wrap;
Query OK, 0 rows affected (0.05 sec)

2、还有一个很精简的办法。

查找重复的，并且除掉最小的那个。

代码如下	复制代码
delete users_groups as a from users_groups as a, ( select *,min(id) from users_groups group by uid having count(1) > 1 ) as b where a.uid = b.uid and a.id > b.id; (7 row(s)affected) (0 ms taken) query result(7 records) id uid gid 1 11 502 2 107 502 3 100 503 4 110 501 5 112 501 6 104 502 9 102 501

3、现在来看一下这两个办法的效率。
运行一下以下SQL 语句

代码如下

复制代码

create index f_uid on users_groups(uid);
explain select * from users_groups group by uid having count(1) > 1 union all
select * from users_groups group by uid having count(1) = 1;
explain select * from users_groups as a,
(
select *,min(id) from users_groups group by uid having count(1) > 1
) as b
where a.uid = b.uid and a.id > b.id;
query result(3 records)
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY users_groups index (NULL) f_uid 4 (NULL) 14
2 UNION users_groups index (NULL) f_uid 4 (NULL) 14
(NULL) UNION RESULT <union1,2> ALL (NULL) (NULL) (NULL) (NULL) (NULL)

query result(3 records)
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY <derived2> ALL (NULL) (NULL) (NULL) (NULL) 4
1 PRIMARY a ref PRIMARY,f_uid f_uid 4 b.uid 1 Using where
2 DERIVED users_groups index (NULL) f_uid 4 (NULL) 14

很明显的第二个比第一个扫描的函数要少。

当没有创建表或创建索引权限的时候

创建一个新表，然后将原表中不重复的数据插入新表：

代码如下

复制代码

mysql> create table demo_new as select * from demo group by site;
Query OK, 3 rows affected (0.19 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo           |
| demo_new       |
+----------------+
2 rows in set (0.00 sec)

mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
| 1 | http://www.hzhuti.com |
| 2 | http://zzzyk.com        |
| 3 | http://www.zzzyk.com |
| 4 | http://www.hzhuti.com |
| 5 | http://www.zzzyk.com |
+----+------------------------+
5 rows in set (0.00 sec)

mysql> select * from demo_new order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
| 1 | http://www.hzhuti.com |
| 2 | http://zzzyk.com        |
| 3 | http://www.zzzyk.com |
+----+------------------------+
3 rows in set (0.00 sec)

然后将原表备份，将新表重命名为当前表：

代码如下

复制代码

补充：数据库,mysql教程