select ta.a,ta.b,ta.c,ta.d,tb.r from 
(
select 'aa' a,'aa' b,'aa' c,'aa' d from dual union all
select 'bb','bb','bb','bb' from dual union all
select 'cc','cc','cc','cc' from dual union all
select 'dd','dd','dd','dd' from dual 
) ta
left join (
select * from 
(
select 'aa' a,'aa' b,'a' r from dual union all
select 'bb','bb','b' from dual union all
select 'cc','cc','c' from dual union all
select 'dd','dd','d' from dual 
) ) tb on ta.a=tb.a and ta.b=tb.b

-------------------------------------
       A    B    C    D    R
1    aa    aa    aa    aa    a
2    bb    bb    bb    bb    b
3    cc    cc    cc    cc    c
4    dd    dd    dd    dd    d