请问sql语句怎么写

[forgetbefore]

with t as
(
     select 1 id,'a' r1,'b' r2 from dual
     union all
     select 2 id,'b' r1,'a' r2 from dual
     union all
     select 3 id,'c' r1,'d' r2 from dual
     union all
     select 4 id,'d' r1,'c' r2 from dual
)

select t.id,t.r1,t.r2 FROM t

查询结果：
id r1 r2
1 b a
2 a b
3 d c
4 c d

请问怎么把交叉重复的 去掉，比如1和2重复 要去掉一条，3和4重复 要去掉一条，
求高手。

[fw0124]

with t as
(
     select 1 id,'a' r1,'b' r2 from dual
     union all
     select 2 id,'b' r1,'a' r2 from dual
     union all
     select 3 id,'c' r1,'d' r2 from dual
     union all
     select 4 id,'d' r1,'c' r2 from dual
)
select max(id), r1, r2 from
(select id, greatest(r1,r2) r1, least(r1,r2) r2 FROM t)
group by r1,r2;

[tobarman]

with t as
(
     select 1 id,'a' r1,'b' r2 from dual
     union all
     select 2 id,'b' r1,'a' r2 from dual
     union all
     select 3 id,'c' r1,'d' r2 from dual
     union all
     select 4 id,'d' r1,'c' r2 from dual
)
SELECT b.id,b.r1,b.r2 FROM
(
       SELECT a.*, row_number() over (PARTITION BY  ascii(a.r1)+ascii(a.r2)ORDER BY  ID  ) num 
       FROM t a
) b
WHERE b.num =1