Oracle:sql语句查询没有重复的记录数目

1、通过创建临时表

可以把数据先导入到一个临时表中,然后删除原表的数据,再把数据导回原表,SQL语句如下:

creat table tbl_tmp (select distinct* from tbl);
truncate table tbl;//清空表记录
insert into tbl select * from tbl_tmp;
//将临时表中的数据插回来。

这种方法可以实现需求,但是很明显,对于一个千万级记录的表,这种方法很慢,在生产系统中,这会给系统带来很大的开销,不可行。

2、利用rowid

在oracle中,每一条记录都有一个rowid,rowid在整个数据库中是唯一的,rowid确定了每条记录是oracle中的哪一个数据文件、块、行上。在重复的记录中,可能所有列的内容都相同,但rowid不会相同。SQL语句如下:

delete from tbl where rowid in (
select a.rowid from tbl a, tbl b where a.rowid>b.rowid
and a.col1=b.col1 and a.col2 = b.col2)

如果已经知道每条记录只有一条重复的,这个sql语句适用。但是如果每条记录的重复记录有N条,这个N是未知的,就要考虑适用下面这种方法了。

3、利用max或min函数

这里也要使用rowid,与上面不同的是结合max或min函数来实现。SQL语句如下

delete from tbl awhere rowid not in (
select max(b.rowid)
from tbl b where a.col1=b.col1 and a.col2 = b.col2);
//这里max使用min也可以
或者用下面的语句
delete from tbl awhere rowid<(select max(b.rowid)
from tbl b where a.col1=b.col1 and a.col2 = b.col2);
//这里如果把max换成min的话,前面的where子句中需要把"<"改为">"

跟上面的方法思路基本是一样的,不过使用了group by,减少了显性的比较条件,提高效率。SQL语句如下:

deletefrom tbl where rowid not in (select max(rowid)
from tbl tgroup by t.col1, t.col2);
delete from tbl where (col1, col2) in
(select col1,col2 from tbl group bycol1,col2
havingcount(*) >1)
and rowid not in (select nin(rowid) from tbl
group by col1,col2 having count(*) >1)

还有一种方法,对于表中有重复记录的记录比较少的,并且有索引的情况,比较适用。假定col1,col2上有索引,并且tbl表中有重复记录的记录比较少,SQL语句如下4、利用group by,提高效率

4、Group By方法

一、查数据
Select count(Num) ,max(Name) from student –列出重复记录,并列出属性
Group By Name
Having count(Num) >1 –按Name分组后找出表中Num列重复,即出现次数大于1
二、删数据
Delete from student Group By Name Having count(Num) > 1
(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)

select g.country,count(DISTINCT(g.GENE_ID)) as gene_count from (select p.gene_id,upper(t.country) as country
        from medline_citation t,tb_gene_2pubmed p
        where t.pmid=p.pubmed_id and t.country is not null ) g group by g.country

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