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。