mysql中重複資料刪除記錄sql語句

重複資料刪除記錄方法一：

1. 建立一個暫存資料表

代碼如下	複製代碼
create table tmp as select * from youtable group by  name（name為不希望有重複的列）

2. 刪除原來的表

代碼如下	複製代碼
drop table youtable

3. 重新命名表

代碼如下	複製代碼
alter table tmp rename youtable

但是這個方法有個問題，由暫存資料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。
重複資料刪除記錄方法二：

1. 建立一個暫存資料表

代碼如下	複製代碼
CREATE TABLE tmp AS SELECT * FROM youtable GROUP BY name（name為不希望有重複的列）

2. 清空原來的表

代碼如下	複製代碼
TRUNCATE TABLE youtable

3. 把暫存資料表插入到youtable

代碼如下	複製代碼
INSERT INTO tablename SELECT  * FROM temp

4. 刪除暫存資料表

代碼如下	複製代碼
DROP TABLE  temp

重複資料刪除記錄方法三：

代碼如下	複製代碼
delete table where ID not in(select min(ID) from table group by name(name:重複的欄位))

重複資料刪除記錄方法四：

具體實現如下：

代碼如下

複製代碼

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

複製代碼

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。

尋找重複的，並且除掉最小的那個。

代碼如下	複製代碼
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 陳述式

代碼如下

複製代碼

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。

當沒有建立表或建立索引許可權的時候

建立一個新表，然後將原表中不重複的資料插入新表：

代碼如下

複製代碼

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，將新表重新命名為當前表：

代碼如下

複製代碼

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)