with a as 
(select 1 a1, 1 a2 from dual
union 
select 2 a1, 2 a2 from dual
union 
select 3 a1, 3 a2 from dual),
 b as 
(select 1 b1, 1 b2 from dual
union 
select 2 b1, 2 b2 from dual
union 
select 4 b1, 4 b2 from dual)
select * from a,b

col1 col2 
1      1
2      2
3      3
4      4

with a as 
(select 1 a1, 1 a2 from dual
union 
select 2 a1, 2 a2 from dual
union 
select 3 a1, 3 a2 from dual),
 b as 
(select 1 b1, 1 b2 from dual
union 
select 2 b1, 2 b2 from dual
union 
select 4 b1, 4 b2 from dual)
select * from a union
select * from b;

with a as 
(select 1 a1, 1 a2 from dual
union 
select 2 a1, 2 a2 from dual
union 
select 3 a1, 3 a2 from dual),
 b as 
(select 1 b1, 1 b2 from dual
union 
select 2 b1, 2 b2 from dual
union 
select 4 b1, 4 b2 from dual)
select nvl(a1,b1) col1,nvl(a2,b2) col2
from a 
full outer join b
on b.b1 = a.a1
and b.b2 = a.a2
order by col1,col2;

      COL1       COL2
---------- ----------
         1          1 
         2          2 
         3          3 
         4          4 

SQL> with a as
  2  (select 1 a1, 1 a2 from dual
  3  union
  4  select 2 a1, 2 a2 from dual
  5  union
  6  select 3 a1, 3 a2 from dual),
  7   b as
  8  (select 1 b1, 1 b2 from dual
  9  union
 10  select 2 b1, 2 b2 from dual
 11  union
 12  select 4 b1, 4 b2 from dual)
 13  select * from a,b
 14  /
        A1         A2         B1         B2
---------- ---------- ---------- ----------
         1          1          1          1
         1          1          2          2
         1          1          4          4
         2          2          1          1
         2          2          2          2
         2          2          4          4
         3          3          1          1
         3          3          2          2
         3          3          4          4

9 rows selected.