select
 *
from (
  select
    a.pk1
  from
     table a,b
  where 
     a.pk1 = b.pk1 (+)
) a_outter_joined,b
a_outter_joined.pk2 = b.pk2 (+) and
(a_outter_joined.pk1 = b.pk1 or a_outter_joined.pk1 = null )