MYSQL的中删除重复记录的方法和代码(2)_MySQL
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
query result(3 records)
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY
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。
推荐阅读
-
MYSQL的中删除重复记录的方法和代码(2)_MySQL
-
Windows Server 2008 R2和2012中PHP连接MySQL过慢的解决方法
-
在C#和MySQL中存取中文字符时避免乱码的方法
-
MySQL中删除重复数据的简单方法
-
在MySQL中创建带有IN和OUT参数的存储过程的方法
-
使用java web 在jsp文件及Class中连接MySQL和SQLsever 的驱动方法
-
使用java web 在jsp文件及Class中连接MySQL和SQLsever 的驱动方法
-
用java实现对MySql数据库中数据的读取、插入、修改和删除
-
mysql 中存在null和空时创建唯一索引的方法
-
mysql 中存在null和空时创建唯一索引的方法