> then, is it possible to construct a join to get:
>
> 1,2,3,-,-,-
> 4,5,6,-,-,-
> -,-,-,a,b,c
> -,-,-,d,e,f
>
> where - means NULL?

You can try this:

select
*
from
table1
full outer join table2 on ( 1 <> 1 )