数据库表中数据行去重复
0.起因
我在创建唯一索引时,提示有重复数据,创建失败。于是,我得先去重,然后再创建唯一索引。
1.建表
sql
CREATE TABLE `demo_table` ( `id` int(10) unsigned NOT NULL, `name` char(255) NOT NULL, `email` char(255) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
2.插入数据,并尝试建立唯一索引
sql
INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('1', 'u1', 'u1@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('2', 'u2', 'u2@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('3', 'u3', 'u3@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('4', 'u4', 'u4@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('5', 'u5', 'u5@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('6', 'u6', 'u6@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('7', 'u7', 'u7@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('8', 'u8', 'u8@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('9', 'u9', 'u9@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('10', 'u10', 'u10@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('11', 'u11', 'u11@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('12', 'u12', 'u12@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('13', 'u13', 'u13@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('14', 'u14', 'u14@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('15', 'u15', 'u15@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('16', 'u16', 'u16@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('17', 'u17', 'u17@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('18', 'u18', 'u18@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('19', 'u19', 'u19@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('20', 'u20', 'u20@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('21', 'u21', 'u1@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('22', 'u22', 'u2@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('23', 'u23', 'u3@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('24', 'u24', 'u4@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('25', 'u25', 'u5@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('26', 'u26', 'u1@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('27', 'u27', 'u2@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('28', 'u28', 'u3@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('29', 'u29', 'u1@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('30', 'u30', 'u2@email.com');
创建唯一索引
sql
CREATE UNIQUE INDEX idx_email ON demo_table(email);
执行结果:ERROR 1062 (23000): Duplicate entry 'u2@email.com' for key 'idx_email'
提示有重复数据,导致创建唯一索引失败。
3.看看我们插入的数据
sql
select * from demo_table;
执行结果:
sql
+----+------+---------------+ | id | name | email | +----+------+---------------+ | 1 | u1 | u1@email.com | | 2 | u2 | u2@email.com | | 3 | u3 | u3@email.com | | 4 | u4 | u4@email.com | | 5 | u5 | u5@email.com | | 6 | u6 | u6@email.com | | 7 | u7 | u7@email.com | | 8 | u8 | u8@email.com | | 9 | u9 | u9@email.com | | 10 | u10 | u10@email.com | | 11 | u11 | u11@email.com | | 12 | u12 | u12@email.com | | 13 | u13 | u13@email.com | | 14 | u14 | u14@email.com | | 15 | u15 | u15@email.com | | 16 | u16 | u16@email.com | | 17 | u17 | u17@email.com | | 18 | u18 | u18@email.com | | 19 | u19 | u19@email.com | | 20 | u20 | u20@email.com | | 21 | u21 | u1@email.com | | 22 | u22 | u2@email.com | | 23 | u23 | u3@email.com | | 24 | u24 | u4@email.com | | 25 | u25 | u5@email.com | | 26 | u26 | u1@email.com | | 27 | u27 | u2@email.com | | 28 | u28 | u3@email.com | | 29 | u29 | u1@email.com | | 30 | u30 | u2@email.com | +----+------+---------------+ 30 rows in set (0.00 sec)
可以看到,我们插入的email字段的确有重复数据。当然,我们是故意的。
4.使用group by看看email字段完全不重复的数据。为了让结果看起来比较顺眼,我们使用order by对结果排一下序,这个不是必选项啦。
sql
select * from demo_table group by email order by id;
执行结果:
sql
+----+------+---------------+ | id | name | email | +----+------+---------------+ | 1 | u1 | u1@email.com | | 2 | u2 | u2@email.com | | 3 | u3 | u3@email.com | | 4 | u4 | u4@email.com | | 5 | u5 | u5@email.com | | 6 | u6 | u6@email.com | | 7 | u7 | u7@email.com | | 8 | u8 | u8@email.com | | 9 | u9 | u9@email.com | | 10 | u10 | u10@email.com | | 11 | u11 | u11@email.com | | 12 | u12 | u12@email.com | | 13 | u13 | u13@email.com | | 14 | u14 | u14@email.com | | 15 | u15 | u15@email.com | | 16 | u16 | u16@email.com | | 17 | u17 | u17@email.com | | 18 | u18 | u18@email.com | | 19 | u19 | u19@email.com | | 20 | u20 | u20@email.com | +----+------+---------------+ 20 rows in set (0.00 sec)
可以发现,表中原本有30条记录,group by得出的结果是只有20条记录,这就说明有10条重复记录。
不相信?好吧,我们再使用having看看吧。group by后,分组记录条数是多少就表示该字段存在多少条。大于1就表示出现了重复。
sql
select *,count(id) from demo_table group by email having count(id)>1 order by id;
执行结果:
sql
+----+------+--------------+-----------+ | id | name | email | count(id) | +----+------+--------------+-----------+ | 1 | u1 | u1@email.com | 4 | | 2 | u2 | u2@email.com | 4 | | 3 | u3 | u3@email.com | 3 | | 4 | u4 | u4@email.com | 2 | | 5 | u5 | u5@email.com | 2 | +----+------+--------------+-----------+ 5 rows in set (0.00 sec)
这下子,我们不仅仅知道重复记录的条数是10条,而且还知道是哪些email出现了重复,并且出现了多次数。
还不相信?好吧,那你自己数吧
5.删除重复数据
我们只保留id最小的数据,其他重复的都给删掉。
sql
delete from demo_table where id not in (select min(id) from demo_table group by email);
执行结果:ERROR 1093 (HY000): You can't specify target table 'demo_table' for update in FROM clause
貌似是MySQL有保护机制,不可以这样操作。
题外话:
如果不是纯使用SQL方式处理,而是通过编程处理,可以把select min(id) from demo_table group by email查询出来的id结果集保存在变量中,然后再执行delete from demo_table where id not id结果集。有一点需要注意的是,可能id结果集很大很大,保存在变量中会吃内存。
6.把需要保留的数据id存在临时表中,然后删除重复数据。
sql
create table tmp_table as select min(id) from demo_table group by email;
执行结果:Query OK, 20 rows affected (0.43 sec)
Records: 20 Duplicates: 0 Warnings: 0
看看我们创建的临时表:
sql
select * from tmp_table;
执行结果:
sql
+---------+ | min(id) | +---------+ | 10 | | 11 | | 12 | | 13 | | 14 | | 15 | | 16 | | 17 | | 18 | | 19 | | 1 | | 20 | | 2 | | 3 | | 4 | | 5 | | 6 | | 7 | | 8 | | 9 | +---------+ 20 rows in set (0.00 sec)
这就是我们要保留的记录的id列表。
删除重复数据:
sql
delete from demo_table where id not in (select * from tmp_table);
执行结果:Query OK, 10 rows affected (0.05 sec)
让我们再看一眼demo_table表吧:
sql
select * from demo_table;
执行结果:
sql
+----+------+---------------+ | id | name | email | +----+------+---------------+ | 1 | u1 | u1@email.com | | 2 | u2 | u2@email.com | | 3 | u3 | u3@email.com | | 4 | u4 | u4@email.com | | 5 | u5 | u5@email.com | | 6 | u6 | u6@email.com | | 7 | u7 | u7@email.com | | 8 | u8 | u8@email.com | | 9 | u9 | u9@email.com | | 10 | u10 | u10@email.com | | 11 | u11 | u11@email.com | | 12 | u12 | u12@email.com | | 13 | u13 | u13@email.com | | 14 | u14 | u14@email.com | | 15 | u15 | u15@email.com | | 16 | u16 | u16@email.com | | 17 | u17 | u17@email.com | | 18 | u18 | u18@email.com | | 19 | u19 | u19@email.com | | 20 | u20 | u20@email.com | +----+------+---------------+ 20 rows in set (0.00 sec)
好的,重复的数据已经被我们成功干掉了。大功告成。等等,我们还得把那个已经没有用处的临时表给删除。貌似有点卸磨杀驴的感觉啊。
sql
drop table tmp_table;
执行结果:Query OK, 0 rows affected (0.12 sec)
7.总结一下吧
其实就三条SQL语句:
sql
create table tmp_table as select min(id) from demo_table group by email; delete from demo_table where id not in (select * from tmp_table); drop table tmp_table;
可能有童鞋要问,如果要去重的不是一个字段,而是多个怎么办?比如,某2条记录的email相同并且name也相同,就算为重复。
处理方式:
sql
group by email,name
再说说数据列去重复吧
1.故意插入一些列重复的数据
sql
INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('31', '31@email.com', '31@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('32', '32@email.com', '32@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('33', '33@email.com', '33@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('34', '34@email.com', '34@email.com'); INSERT INTO `test`.`demo_table` (`id`, `name`, `email`) VALUES ('35', '35@email.com', '35@email.com');
查找表中name字段和email字段相同的数据:
sql
select * from demo_table as a, demo_table as b where a.id=b.id and a.name=b.email;
执行结果:
sql
+----+--------------+--------------+----+--------------+--------------+ | id | name | email | id | name | email | +----+--------------+--------------+----+--------------+--------------+ | 31 | 31@email.com | 31@email.com | 31 | 31@email.com | 31@email.com | | 32 | 32@email.com | 32@email.com | 32 | 32@email.com | 32@email.com | | 33 | 33@email.com | 33@email.com | 33 | 33@email.com | 33@email.com | | 34 | 34@email.com | 34@email.com | 34 | 34@email.com | 34@email.com | | 35 | 35@email.com | 35@email.com | 35 | 35@email.com | 35@email.com | +----+--------------+--------------+----+--------------+--------------+ 5 rows in set (0.00 sec)
重复记录的id都已经找出来了,删除就参考上述方式处理了。