[pong]

select kd.IDKunde,
       kd.VollerName,
       ko.Datum,
       ko.Kontostand
from tblKunden kd,
     tblKontostand ko
where ko.IDKunde = kd.IDKunde
  and ko.Datum = (select ko2.Datum
                  from tblKontostand ko2
                  where ko2.IDKunde = kd.IDKunde
                    and ko2.IDKontostand = ko.IDKontostand
                  order by ko2.datum desc
                  fetch first 1 row only
                 )
;

Alternative zur Aggregatfunktion.

pong