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。