MySql-如何查询删除数据表重复记录
name, email, COUNT(*)
name, email
COUNT(*) & 1
重点来了，查询容易，那应该如何删除重复记录呢？
+-------+------------------+------+-----+---------+----------------+
| Field | Type
| Null | Key | Default | Extra
+-------+------------------+------+-----+---------+----------------+
| int(11) unsigned | NO
| PRI | NULL
| auto_increment |
| varchar(100)
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql& select * f
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
3 | http://www.ShuoWen.org |
5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)
当没有创建表或创建索引权限的时候，可以用下面的方法：
如果你要删除较旧的重复记录，可以使用下面的语句：
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 * f
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)
如果你要删除较新的重复记录，可以使用下面的语句：
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 * f
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
5 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)
你可以用下面的语句先确认将被删除的重复记录：
mysql& SELECT a.*
-& FROM demo a, demo b
-& WHERE a.id & b.id
-& AND (a.site = b.site);
+----+------------------------+
| id | site
+----+------------------------+
3 | http://www.ShuoWen.org |
+----+------------------------+
2 rows in set (0.00 sec)
如果有创建索引的权限，可以用下面的方法：
在表上创建唯一键索引：
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 * f
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)
重复记录被删除后，如果需要，可以删除索引：
mysql& alter table
Query OK, 3 rows affected (0.37 sec)
Records: 3
Duplicates: 0
Warnings: 0
如果有创建表的权限，可以用下面的方法：
创建一个新表，然后将原表中不重复的数据插入新表：
mysql& create table demo_new as select * fro
Query OK, 3 rows affected (0.19 sec)
Records: 3
Duplicates: 0
Warnings: 0
+----------------+
| Tables_in_test |
+----------------+
| demo_new
+----------------+
2 rows in set (0.00 sec)
mysql& select * f
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
3 | http://www.ShuoWen.org |
5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)
mysql& select * from demo_
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)
然后将原表备份，将新表重命名为当前表：
mysql& rename table demo to demo_old, demo_
Query OK, 0 rows affected (0.04 sec)
+----------------+
| Tables_in_test |
+----------------+
| demo_old
+----------------+
2 rows in set (0.00 sec)
mysql& select * f
+----+------------------------+
| id | site
+----+------------------------+
2 | http://YITU.org
3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)
注意：使用这种方式创建的表会丢失原表的索引信息！
+-------+------------------+------+-----+---------+-------+
| Field | Type
| Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| int(11) unsigned | NO
| varchar(100)
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
如果要保持和原表信息一致，你可以使用 sh 来查看原表的创建语句，然后使用原表的创建语句创建新表，接着使用 insert & select 语句插入数据，再重命名表即可。
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display: 'inlay-fix'11:18 提问
MySQL 删除重复数据（两个字段）
表full中userID和movieID两个字段确定一条唯一的数据。
现在表中有少量重复数据和大量非重复数据。
重复数据中，preference列和timestamp列不为空的这些，需要保留；preference列和timestamp列为空的这些需要删除，应该怎么办？
初学者，希望答案详细:) 感激不尽
+------------+---------+------+-----+---------+-------+
| Null | Key | Default | Extra |
+------------+---------+------+-----+---------+-------+
| int(11) | NO
| int(11) | NO
| preference | int(11) | YES
| timestamp
| int(11) | YES
+------------+---------+------+-----+---------+-------+
按赞数排序
先建一个表，用distinct查询不重复的数据放到新建的表里面，然后把原表数据清空，再把刚才新表的数据插到原表里
然后把数据导入到新表中：
INSERT INTO table2 SELECT distinct * FROM table1;
你试一下，看可以不？
step 1: 复制table为table_copy
step 2: delete from table where UserID is NULL and MovieID is NULL and (UserID,MovieID) in (select t1.UserID,t1.MovieID from table_copy as t1,table_copy as t2 where t1.UserID = t2.UserID and t1.MovieID = t2.MovieID and not (t1.preference = t2.preference and t1.timestamp = t2.timestamp));
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