MySQL 數(shù)據(jù)庫中刪除重復(fù)記錄的方法總結(jié)

演示數(shù)據(jù)

表結(jié)構(gòu)：

mysql> desc demo;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| site  | varchar(100)     | NO   | MUL |         |                |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

數(shù)據(jù)：

mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
|  4 | http://www.CodeBit.cn  |
|  5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)

當(dāng)沒有創(chuàng)建表或創(chuàng)建索引權(quán)限的時(shí)候，可以用下面的方法：

如果你要?jiǎng)h除較舊的重復(fù)記錄，可以使用下面的語句：

mysql> delete from a
    -> using demo as a, demo as b
    -> where (a.id > b.id)
    -> and (a.site = b.site);
Query OK, 2 rows affected (0.12 sec)

mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

如果你要?jiǎng)h除較新的重復(fù)記錄，可以使用下面的語句：

mysql> delete from a
    -> using demo as a, demo as b
    -> where (a.id < b.id)
    -> and (a.site = b.site);
Query OK, 2 rows affected (0.12 sec)

mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  2 | http://YITU.org        |
|  4 | http://www.CodeBit.cn  |
|  5 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

你可以用下面的語句先確認(rèn)將被刪除的重復(fù)記錄：

mysql> SELECT a.*
    -> FROM demo a, demo b
    -> WHERE a.id > b.id
    -> AND (a.site = b.site);
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
2 rows in set (0.00 sec)

如果有創(chuàng)建索引的權(quán)限，可以用下面的方法：

在表上創(chuàng)建唯一鍵索引：

mysql> alter ignore table demo add unique index ukey (site);
Query OK, 5 rows affected (0.46 sec)
Records: 5  Duplicates: 2  Warnings: 0

mysql> select * from demo order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

重復(fù)記錄被刪除后，如果需要，可以刪除索引：

mysql> alter table demo drop index ukey;
Query OK, 3 rows affected (0.37 sec)
Records: 3  Duplicates: 0  Warnings: 0

如果有創(chuàng)建表的權(quán)限，可以用下面的方法：

創(chuàng)建一個(gè)新表，然后將原表中不重復(fù)的數(shù)據(jù)插入新表：

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.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
|  4 | http://www.CodeBit.cn  |
|  5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)

mysql> select * from demo_new order by id;
+----+------------------------+
| id | site                   |
+----+------------------------+
|  1 | http://www.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

然后將原表備份，將新表重命名為當(dāng)前表：

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.CodeBit.cn  |
|  2 | http://YITU.org        |
|  3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

注意：使用這種方式創(chuàng)建的表會(huì)丟失原表的索引信息！

mysql> desc demo;
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id    | int(11) unsigned | NO   |     | 0       |       |
| site  | varchar(100)     | NO   |     |         |       |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

如果要保持和原表信息一致，你可以使用 show create table demo; 來查看原表的創(chuàng)建語句，然后使用原表的創(chuàng)建語句創(chuàng)建新表，接著使用 insert … select 語句插入數(shù)據(jù)，再重命名表即可。