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  • mysql去重, 把url重复且区为空的中去掉、统计重复数据、、结果集去重合并成一行

    delete from 表名 where id not in (select d.id from (SELECT id FROM 表名 GROUP BY c1,c2,c3,c4)as d)

    #去重复，把url重复，且区为空的中去掉。
    select * from TABLE where 
    url in 
    (select u.url from (select * from TABLE where id not in (select d.id from (SELECT id FROM TABLE GROUP BY url)as d)) as u) 

    and qu like "";

    二、删除/去除重复记录

    select *, count(distinct name) from table group by name

    SELECT shortname,COUNT(*) FROM student GROUP BY shortname HAVING COUNT(*) > 1;

    select user_name,count(*) as count from user group by user_name having count>1;

    创建临时表：

    create table tb2 select c1,c2 from tb1 group by c1,c2;

    drop table tb1;

    alter table tb2 rename to tb1;


    三、统计重复数据：
    select c1,c2
    from tb1
    group by c1,c2
    having count(*)>1;
    除了GROUP BY 来读取数据表中不重复的数据，使用DISTINCT关键字来过滤。

    用distinct:

    select distinct c1,c2 from table;

     

    四、防止表中出现重复数据：

    在MySQL数据表中设置指定的字段为PRIMARY KEY索引/UNIQUE索引）来保证数据的唯一性。
    create table tb1 (
    c1 char(16) not null,
    c2 char(16) not null,
    c3 char(10),
    primary key(c1,c2)
    )

    或者：

    create table tb1
    (
    c1 char(20) not null,
    c2 char(20) not null,
    c3 char(10),
    unique(c1,c2)
    )

    mysql查询表里的重复数据方法

     更新时间：2017年05月20日 13:51:47   投稿：mdxy-dxy    我要评论

    这篇文章主要介绍了mysql查询表里的重复数据方法,需要的朋友可以参考下
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    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

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    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：

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    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：

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    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：

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    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：

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

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    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

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    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。

     

     

    MySQL之——查询重复记录、删除重复记录方法大全

    查找所有重复标题的记录：

    SELECT * FROM t_info a WHERE ((SELECT COUNT(*) FROM t_info WHERE Title = a.Title) > 1) ORDER BY Title DESC

    一、查找重复记录

    1、查找全部重复记录

    Select * From 表 Where 重复字段 In (Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1)

    2、过滤重复记录(只显示一条) 

    Select * From HZT Where ID In (Select Max(ID) From HZT Group By Title)
    注：此处显示ID最大一条记录

    二、删除重复记录

    1、删除全部重复记录（慎用） 

    Delete 表 Where 重复字段 In (Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1)

    2、保留一条（这个应该是大多数人所需要的 ^_^）

    Delete HZT Where ID Not In (Select Max(ID) From HZT Group By Title)
    注：此处保留ID最大一条记录

    三、举例

    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rowid最小的记录
    delete from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) and rowid not in (select min(rowid) 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)
    


    四、补充
    

    有两个以上的重复记录，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 
      

    1. 

       

      select distinct * into #Tmp from tableName
      





      






      2. 

    2. 

       

      drop table tableName
      





      






      3. 

    3. 

       

      select * into tableName from #Tmp
      





      





      4. 

    4. 

      

      drop table #Tmp
      






      






      5. 






    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)
    

     
    






    





    mysql 结果集去重复值并合并成一行
    

    +——+——+
    | id| name |　tag |
    +——+——+
    |1 | 10| A
    |1 | 20| A
    |1 | 20| C
    |2 | 20| C
    |3 | 200 | B
    |3 | 500 | A
    +——+——+
    


    以id分组，把name字段的值打印在一行，逗号分隔(默认)
    select id,group_concat(name) from student group by id;  
    

    以id分组，把name字段的值打印在一行，分号分隔：
    select id,group_concat(name separator ’;’) from aa group by id;  
    

    以id分组，把去冗余的name字段的值打印在一行，逗号分隔：
    select id,group_concat(distinct name) from aa group by id;  
    


    
    
                
    
                
              
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                原文地址：https://www.cnblogs.com/timssd/p/5827575.html
              
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