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删除重复的数据保留一条
MySQL查询表内重复记录
查询及删除重复记录的⽅法
(⼀)
1、查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)NT(*) > 1) AND rowid NOT IN (SELECT MIN(rowid) FROM table GROUP BY mail_id,phone HAVING COUNT(*)>1);
5、查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)stions GROUP BY questions_title,questions_scope HAVING COUNT(*) > 1) AND question_id NOT IN (SELECT MIN(q 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 DELETE FROM questions WHERE question_id IN (SELECT question_id FROM tmp);
DROP TABLE tmp;
查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为要查的重复字段.
count⽤来判断⼤于⼀的才是重复的.
user_table为要查的表名.
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，到⼀个适合⾃⼰⽹站的查询语句。

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