查询
SELECT
name, email, COUNT(*)
FROM
users
GROUP BY
name, email
HAVING
COUNT(*) > 1<strong>重点来了，查询容易，那应该如何删除重复记录呢？</strong>
演示数据<br>
表结构：
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)
<code class="hljs asciidoc">数据：
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)
<code class="hljs asciidoc"><code class="hljs asciidoc">当<strong>没有创建表或创建索引权限</strong>的时候，可以用下面的方法：
<code class="hljs asciidoc"><code class="hljs asciidoc">如果你要<strong>删除较旧的重复记录</strong>，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 | http://www.CodeBit.cn | | 2 | http://YITU.org | | 3 | http://www.ShuoWen.org | +----+------------------------+ 3 rows in set (0.00 sec)
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">如果你要<strong>删除较新的重复记录</strong>，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 | +----+------------------------+ | 2 | http://YITU.org | | 4 | http://www.CodeBit.cn | | 5 | http://www.ShuoWen.org | +----+------------------------+ 3 rows in set (0.00 sec)
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">你可以用下面的语句<strong>先确认将被删除的重复记录</strong>：
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)<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc">如果<strong>有创建索引的权限</strong>，可以用下面的方法：
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc">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 | http://www.CodeBit.cn | | 2 | http://YITU.org | | 3 | http://www.ShuoWen.org | +----+------------------------+ 3 rows in set (0.00 sec)
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc">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
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">如果<strong>有创建表的权限</strong>，可以用下面的方法：
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">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 | 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)
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">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 | http://www.CodeBit.cn | | 2 | http://YITU.org | | 3 | http://www.ShuoWen.org | +----+------------------------+ 3 rows in set (0.00 sec)
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">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)
<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">如果要保持和原表信息一致，你可以使用 <strong>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</strong>，接着使用 insert … select 语句插入数据，再重命名表即可。
