자체 해결해보았습니다.

select --a.rn,b.rn,
       nvl(a.a,b.a) a,nvl(a.b,b.b) b,nvl(a.c,b.c) c from
(
    with a as
    ( select 'a' a , 'b1' b , '' c from dual
      union
      select 'a' a , 'b2' b , '' c from dual
      union
      select 'a' a , 'b3' b , '' c from dual
      union
      select 'a' a , 'b4' b , '' c from dual
      union
      select 'b' a , 'b5' b , '' c from dual
      union
      select 'b' a , 'b6' b , '' c from dual
    )
    select a.*,row_number() over(partition by a.a order by a.a,a.b,a.c asc) rn from a
) a
full outer join
(
    with b as
    (
       select 'a' a , '' b , 'c1' c from dual
      union
      select 'a' a , '' b , 'c2' c from dual
      union
      select 'b' a , '' b , 'c3' c from dual
      union
      select 'b' a , '' b , 'c4' c from dual
      union
      select 'b' a , '' b , 'c5' c from dual
    )
    select b.*,row_number() over(partition by b.a order by b.a,b.b,b.c asc) rn from b
) b
on a.a = b.a
and a.rn = b.rn
order by nvl(a.a,b.a),nvl(a.b,b.b),nvl(a.c,b.c)
;
 

위 SQL로 원하는 결과는 얻었는데, 정확하고 효율적인 SQL인지는 정확한 판단이 서질 않네요 ^^;

많은 고수님들의 조언 부탁드립니다. ^^

꾸벅..~~!!