SQL & PL/SQL

Hi Gurus!

I have a table ff, that has 2 columns id_1 and id_2.

with ff as
(
select 1 id_1, 1    id_2 from dual union all
select 2 id_1, null id_2 from dual union all
select 3 id_1, null id_2 from dual union all
select 4 id_1, 2    id_2 from dual union all
select 5 id_1, 3    id_2 from dual union all
select 6 id_1, null id_2 from dual union all
select 7 id_1, 4    id_2 from dual 

)select * from ff

ID_1	ID_2
------------
1	1
2	
3	
4	2
5	3
6	
7	4

Desired output:

ID_1	ID_2
------------
1	1
2	1
3	1
4	2
5	3
6	3
7	4

Hope this makes sense

Thanks