select
      "Konto",
      case when "Soll" is null then 0 else "Soll" end,
      case when "Haben" is null then 0 else "Haben" end
from
((select ktnr as "Konto", sum(dm) as "Soll" from ktp
      where dat <= '2004-12-31'
      group by "Konto") as t1
natural full join
(select gkto as "Konto", sum(dm) as "Haben" from ktp
      where dat <= '2004-12-31'
      group by "Konto") as t2);