刪除重復記錄方法一:
1. 新建一個臨時表
代碼如下 復制代碼create table tmp as select * from youtable group by name(name為不希望有重復的列)
2. 刪除原來的表
代碼如下 復制代碼drop table youtable
3. 重命名表
代碼如下 復制代碼alter table tmp rename youtable
但是這個方法有個問題,由臨時表轉變過來的最終表,其表結構會和原來的不一致,需要手工更改。這個問題,待解決。
刪除重復記錄方法二:
1. 新建一個臨時表
代碼如下 復制代碼CREATE TABLE tmp AS SELECT * FROM youtable GROUP BY name(name為不希望有重復的列)
2. 清空原來的表
代碼如下 復制代碼TRUNCATE TABLE youtable
3. 把臨時表插入到youtable
代碼如下 復制代碼INSERT INTO tablename SELECT * FROM temp
4. 刪除臨時表
代碼如下 復制代碼DROP TABLE temp
刪除重復記錄方法三:
代碼如下 復制代碼delete table where ID not in(select min(ID) from table group by name(name:重復的字段))
刪除重復記錄方法四:
具體實現如下:
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內容:
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)
根據一位兄弟的建議修改。
代碼如下 復制代碼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、還有一個很精簡的辦法。
查找重復的,並且除掉最小的那個。
代碼如下 復制代碼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、現在來看一下這兩個辦法的效率。
運行一下以下SQL 語句
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
很明顯的第二個比第一個掃描的函數要少。
當沒有創建表或創建索引權限的時候
創建一個新表,然後將原表中不重復的數據插入新表:
代碼如下 復制代碼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.111cn.net |
| 2 | http://111cn.net |
| 3 | http://www.111cn.net |
| 4 | http://www.111cn.net |
| 5 | http://www.111cn.net |
+----+------------------------+
5 rows in set (0.00 sec)
mysql> select * from demo_new order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.111cn.net |
| 2 | http://111cn.net |
| 3 | http://www.111cn.net |
+----+------------------------+
3 rows in set (0.00 sec)
然後將原表備份,將新表重命名為當前表:
代碼如下 復制代碼mysql> rename table demo to demo_old, demo_new to demo;
Query OK, 0 rows affected (0.04 sec)
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo |
| demo_old |
+----------------+
2 rows in set (0.00 sec)
mysql> select * from demo order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.111cn.net |
| 2 | http://111cn.net |
| 3 | http://www.111cn.net |
+----+------------------------+
3 rows in set (0.00 sec)