INSERT INTO hk_test(username, passwd) VALUES
('qmf1', 'qmf1'),('qmf2', 'qmf11')
 
delete from hk_test where username='qmf1' and passwd='qmf1'

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;

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

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`;

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

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;