ORACLE 去重

-----------------------------------------------------------------------------模拟数据---------------------------------------------------------------------------
--创建测试数据及表结构

create table tablename(N1 varchar2(10), N2 varchar2(10), N3 varchar2(10));

insert into tablename 1, 2, 3 from dual union allselect 1, 2, 3 from dual union allselect 5, 2, 3 from dual union allselect 10, 20, 30 from dual ;commit;select*from tablename;

--数据集（针对指定列，查出去重后的结果集）
--N1 N2 N3
--1 2 3
--1 2 3
--5 2 3
--10 20 30

-----------------------------------------------------------------------------distinct-----------------------------------------------------------------------------
select distinct t1.* from tablename t1;
N1 N2 N3
10 20 30
1 2 3
5 2 3
--方法局限性很大，因为它只能对全部查询的列做去重。如果我想对N2,N3去重，那我的结果集中就只能有N2,N3列，而不能有N1列。

select distinct t1.N2, N3 from tablename t1
N2 N3
2 3
20 30
--不过它也是最简单易懂的写法。

-----------------------------------------------------------------------------row_number-----------------------------------------------------------------------------
select * from (select t1.*,
row_number() over(partition by t1.N2, t1.N3 order by 1) rn
from tablename t1) t1 where t1.rn = 1;

N1 N2 N3 RN
1 2 3 1
10 20 30 1
--写法上要麻烦不少，但是有更大的灵活性。

--针对指定列，查出所有重复的行
-----------------------------------------------------------------------------count having-----------------------------------------------------------------------------
select * from tablename t
where (t.N2, t.N3) in (select t1.N2, t1.N3
from tablename t1
group by t1.N2, t1.N3
having count(1) > 1)
N1 N2 N3
1 2 3
1 2 3
5 2 3
--要查两次表，效率会比较低。不推荐。
------------------------------------------------------------------------------count over----------------------------------------------------------------------------
select * from (select t1.*,
count(1) over(partition by t1.N2, t1.N3) rn
from tablename t1) t1 where t1.rn > 1;
N1 N2 N3 RN
1 2 3 3
1 2 3 3
5 2 3 3
--只需要查一次表，推荐。