mysql查询表里的重复数据方法

insert into hk_test(username, passwd) values
('qmf1', 'qmf1'),('qmf2', 'qmf11')
 
delete from hk_test where username='qmf1' and passwd='qmf1'

mysql里查询表里的重复数据记录：

先查看重复的原始数据：

场景一：列出username字段有重读的数据

select username,count(*) as count from hk_test group by username having count>1;
 
select username,count(username) as count from hk_test group by username having count(username) >1 order by count desc;

这种方法只是统计了该字段重复对应的具体的个数

场景二：列出username字段重复记录的具体指：

select * from hk_test where username in (select username from hk_test group by username having count(username) > 1)
 
select username,passwd from hk_test where username in ( select username from hk_test group by username having count(username)>1)
 
但是这条语句在mysql中效率太差，感觉mysql并没有为子查询生成临时表。在数据量大的时候，耗时很长时间

解决方法：

于是使用先建立临时表 
 
create table `tmptable` as ( 
select `name` 
from `table` 
group by `name` having count(`name`) >1 
); 
 
然后使用多表连接查询
 
select a.`id`, a.`name` 
from `table` a, `tmptable` t 
where a.`name` = t.`name`; 
 
结果这次结果很快就出来了。
 
用 distinct去重复
 
select distinct a.`id`, a.`name` 
from `table` a, `tmptable` t 
where a.`name` = t.`name`;

场景三：查看两个字段都重复的记录：比如username和passwd两个字段都有重复的记录：

select * from hk_test a
where (a.username,a.passwd) in (select username,passwd from hk_test group by username,passwd having count(*) > 1)

场景四：查询表中多个字段同时重复的记录：

select username,passwd,count(*) from hk_test group by username,passwd having count(*) > 1

mysql查询表内重复记录
 
查询及删除重复记录的方法
(一)
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleid）来判断
select * 
from people
where peopleid in (select peopleid from people group by peopleid having count(peopleid)>1)
 
2、删除表中多余的重复记录，重复记录是根据单个字段（peopleid）来判断，只留有一个记录
delete from people
where peopleid in (select peopleid 
from people group by peopleid having count(peopleid)>1)
and min(id) not 
in (select id from people group by peopleid having count(peopleid)>1)
 
3、查找表中多余的重复记录（多个字段）
select * from vitae a
where (a.peopleid,a.seq) in 
(select peopleid,seq from vitae group by peopleid,seq having count(*)>1)
 
4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录
delete from vitae a
where 
(a.peopleid,a.seq) in (select peopleid,seq from vitae group by peopleid,seq 
having count(*) > 1)
and rowid not in (select min(rowid) from vitae group 
by peopleid,seq having count(*)>1)
 
5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录
select * from vitae a
where 
(a.peopleid,a.seq) in (select peopleid,seq from vitae group by peopleid,seq 
having count(*) > 1)
and rowid not in (select min(rowid) from vitae group 
by peopleid,seq having count(*)>1)
 
(二)
比方说
在a表中存在一个字段“name”，而且不同记录之间的“name”值有可能会相同，现在就是需要查询出在该表中的各记录之间，“name”值存在重复的项；
select name,count(*) from a group by name having count(*) > 1
如果还查性别也相同大则如下:
select name,sex,count(*) from a group by name,sex having count(*) > 1
 
(三)
方法一
declare @max integer,@id integer
declare cur_rows cursor local for select 主字段,count(*) from 表名 group by 主字段 
having count(*) >； 1
open cur_rows
fetch cur_rows into @id,@max
while @@fetch_status=0
begin
select @max = @max -1
set rowcount @max
delete from 表名 where 主字段 = @id
fetch cur_rows into @id,@max
end
close cur_rows
set rowcount 0

select * from tab1 where companyname in( select companyname from tab1 group by companyname having count(*)>1);
-- 129.433ms 
 
select * from tab1 inner join ( select companyname from tab1 group by companyname having count(*)>1) as tab2 using(companyname);
-- 0.482ms
 
方法二
 
　　有两个意义上的重复记录，一是完全重复的记录，也即所有字段均重复的记录，二是部分关键字段重复的记录，比如name字段重复，而其他字段不一定重复或都重复可以忽略。
 
　　1、对于第一种重复，比较容易解决，使用
 
select distinct * from tablename
 
　　就可以得到无重复记录的结果集。
 
　　如果该表需要删除重复的记录（重复记录保留1条），可以按以下方法删除
 
select distinct * into #tmp from tablename
 
drop table tablename
 
select * into tablename from #tmp
 
drop table #tmp
 
　　发生这种重复的原因是表设计不周产生的，增加唯一索引列即可解决。
 
2、这类重复问题通常要求保留重复记录中的第一条记录，操作方法如下
 
　　假设有重复的字段为name,address，要求得到这两个字段唯一的结果集
 
select identity(int,1,1) as autoid, * into #tmp from tablename
 
select min(autoid) as autoid into #tmp2 from #tmp group by name,autoid
 
select * from #tmp where autoid in(select autoid from #tmp2)
 
最后一个select即得到了name，address不重复的结果集（但多了一个autoid字段，实际写时可以写在select子句中省去此列）
 
(四)查询重复
 
select * from tablename where id in (
 
select id from tablename group by id having count(id) > 1)

常用的语句
 
1、查找表中多余的重复记录，重复记录是根据单个字段（mail_id）来判断
 
 代码如下 复制代码 
 select * from table where mail_id in (select mail_id from table group by mail_id having count(mail_id) > 1);
 
 
2、删除表中多余的重复记录，重复记录是根据单个字段（mail_id）来判断，只留有rowid最小的记录
 
 代码如下 复制代码 
delete from table where mail_id in (select mail_id from table group by mail_id having count(mail_id) > 1) and rowid not in (select min(rowid) from table group by mail_id having count(mail_id )>1);
 
 
3、查找表中多余的重复记录（多个字段）
 
 代码如下 复制代码 
select * from table where (mail_id,phone) in (select mail_id,phone from table group by mail_id,phone having count(*) > 1);
 
 
4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录
 
 代码如下 复制代码 
 delete from table where (mail_id,phone) in (select mail_id,phone from table group by mail_id,phone having cou(www.jb51.net)nt(*) > 1) and rowid not in (select min(rowid) from table group by mail_id,phone having count(*)>1);
 
 
5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录
 
 代码如下 复制代码 
select * from table where (a.mail_id,a.phone) in (select mail_id,phone from table group by mail_id,phone having count(*) > 1) and rowid not in (select min(rowid) from table group by mail_id,phone having count(*)>1);
 
 
存储过程
 
declare @max integer,@id integer
 
declare cur_rows cursor local for select 主字段,count(*) from 表名 group by 主字段 having count(*) >； 1
 
open cur_rows
 
fetch cur_rows into @id,@max
 
while @@fetch_status=0
 
begin
 
select @max = @max -1
 
set rowcount @max
 
delete from 表名 where 主字段 = @id
 
fetch cur_rows into @id,@max
 
end
 
close cur_rows
 
set rowcount 0
 
 
 
(一)单个字段
 
1、查找表中多余的重复记录，根据（question_title）字段来判断
 
 代码如下 复制代码 
select * from questions where question_title in (select question_title from people group by question_title having count(question_title) > 1)
 
 
2、删除表中多余的重复记录，根据（question_title）字段来判断，只留有一个记录
 
 代码如下 复制代码 
delete from questions
where peopleid in (select peopleid from people group by peopleid having count(question_title) > 1)
and min(id) not in (select question_id from questions group by question_title having count(question_title)>1) 
 
(二)多个字段
 
删除表中多余的重复记录（多个字段），只留有rowid最小的记录
 
 代码如下 复制代码 
delete from questions where (questions_title,questions_scope) in (select questions_title,questions_scope from que(www.jb51.net)stions group by questions_title,questions_scope having count(*) > 1) and question_id not in (select min(question_id) from questions group by questions_scope,questions_title having count(*)>1)
 
 
用上述语句无法删除，创建了临时表才删的，求各位达人解释一下。
 
 代码如下 复制代码 
create table tmp as select question_id from questions where (questions_title,questions_scope) in (select questions_title,questions_scope from questions group by questions_title,questions_scope having count(*) > 1) and question_id not in (select min(question_id) from questions group by questions_scope,questions_title having count(*)>1);
 
delete from questions where question_id in (select question_id from tmp);
 
drop table tmp;

查找mysql数据表中重复记录
mysql数据库中的数据越来越多,当然排除不了重复的数据,在维护数据的时候突然想到要把多余的数据给删减掉,剩下有价值的数据。

以下sql语句可以实现查找出一个表中的所有重复的记录.
select user_name,count(*) as count from user_table group by user_name having count>1;

参数说明:

user_name为要查找的重复字段.

count用来判断大于一的才是重复的.

user_table为要查找的表名.

group by用来分组

having用来过滤.

把参数换成自己数据表的相应字段参数，可以先在phpmyadmin里面或者navicat里面去运行，看看有哪些数据重复了，然后在数据库里面删除掉，也可以直接将sql语句放到后台读取新闻的页面里面读取出来，完善成查询重复数据的列表，有重复的可以直接删除。

效果如下：

缺点：这种方法的缺点就是当你的数据库里面的数据量很大的时候，效率很低，我用的是navicat测试的，数据量不大，效率很高，当然，网站还有其它查询数据重复的sql语句，举一反三，大家好好研究研究，找到一个适合自己网站的查询语句。