数据库|mysql教程
demo,http,mysql,rows,site
数据库-mysql教程
查询
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SELECT name, email, COUNT(*)FROM usersGROUP BY name, emailHAVINGCOUNT(*) > 1
重点来了，查询容易，那应该如何删除重复记录呢？
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表结构：演示数据
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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)
数据：
mysql> select * from demo order by id;+----+------------------------+| id | site |+----+------------------------+| 1 | || 2 | || 3 | || 4 | || 5 | |+----+------------------------+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 * from demo order by id;+----+------------------------+| id | site |+----+------------------------+| 1 | || 2 | || 3 | |+----+------------------------+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 and (a.site = b.site);Query OK, 2 rows affected (0.12 sec)
mysql> select * from demo order by id;+----+------------------------+| id | site |+----+------------------------+| 2 | || 4 | || 5 | |+----+------------------------+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 |+----+------------------------+| 1 | || 3 | |+----+------------------------+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 * from demo order by id;+----+------------------------+| id | site |+----+------------------------+| 1 | || 2 | || 3 | |+----+------------------------+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 demo drop index ukey;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 * 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 | || 2 | || 3 | || 4 | || 5 | |+----+------------------------+5 rows in set (0.00 sec)
mysql> select * from demo_new order by id;+----+------------------------+| id | site |+----+------------------------+| 1 | || 2 | || 3 | |+----+------------------------+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_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 | || 2 | || 3 | |+----+------------------------+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！
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; 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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