Question

Oracle查询去除重数据

Answer 1

1。用rowid方法

据据oracle带的rowid属性，进行判断，是否存在重复,语句如下：

查数据:

select * from table1 a where rowid
!=(select max(rowid)

from table1 b where a.name1=b.name1 and
a.name2=b.name2......)

删数据：

delete from table1 a where rowid
!=(select max(rowid)

from table1 b where a.name1=b.name1 and
a.name2=b.name2......)

2.group by方法

查数据:

select count(num), max(name) from student --列出重复的记录数，并列出他的name属性

group by num

having count(num) >1 --按num分组后找出表中num列重复，即出现次数大于一次

删数据：

delete from student

group by num

having count(num) >1

这样的话就把所有重复的都删除了。

3.用distinct方法 -对于小的表比较有用

create table table_new as select distinct *
from table1 minux

truncate table table1;

insert into table1 select * from table_new;

Answer 2

1、distinct 关键字的用法：distinct 关键字后面的字段组合去重 distinct 必须

select distinct id from test
结果 ;根据id 去重
select distinct id，name from test
2、group by 分组去重
select id，name from test group by id，name
结果：根据id，name 组合去重
3、row_number ()over(partition by 列 order by 列 asc | desc)方法
3.1 row_number() over(order by column asc) 先对列column按照升序，再为每条记录返回一个序列号
3.2 row_number() over(partition by column1 order by column2 asc) 先按照column1分组，再对分组后的数据根据column2 升序排列

Answer 3

select distinct rowid where 表名

Answer 4

分组后，根据rowid取就可以