MYSQL中删除重复记录的方法和代码

2008-04-21 16:08:23　来源:中国自学编程网　作者:佚名点击:

在实际应用中，很可能会碰到一些需要删除某些字段的重复记录，我现在把我能想到的写下来，望高手们补充。

4 rows in set (0.00 sec)

mysql> create temporary table tmp_wrap select * from users_groups group by uid having count(1) > 1 union all
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。

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

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