sql去除重复语句

1、查找表中多余的重复记录，重复记录是根据单个字段（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) ;




查询表中前5条记录：


select * from 表1 limit 5;  
 
select * from 表1 limit 0,5; 


查询表中第5条到第7条记录：


select * from 表1 limit 4,3; 


随即选取一个记录：


select * from 表1 order by rand() limit 1;  


随即选取n条记录：


select * from 表1 order by rand() limit n;  




MySQL查询结果按某值排序


select * from test where id in(3,1,5);  


select * from test where id in(3,1,5) order by find_in_set(id,'3,1,5');  
select * from test where id in(3,1,5) order by substring_index('3,1,2',id,1);  



sql 单表/多表查询去除重复记录

单表distinct

多表group by

group by 必须放在 order by 和 limit之前，不然会报错

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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);

(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

方法二

＂重复记录＂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 )

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)

运行会产生问题，where(a.peopleId,a.seq)这样的写发是通不过的！！！ 


mysql 重复记录只选取最后一条




# sid相同，create_time不同，取create_time最后一条


1. 使用 NOT EXISTS参数
SELECT id, sid FROM table_name a WHERE NOT EXISTS (SELECT 1 FROM table_name where a.sid = sid AND a.create_time < create_time) 


2. 使用子查询
SELECT id, sid FROM table_name WHERE create_time IN (SELECT max(create_time) FROM table_name group by sid)