select RE_NR,AB_NR,
case
when Fracht=='0.00' then (select Fracht from tabelle b where Fracht !='0.00' and a.AB_NR=b.AB_NR)
else
Fracht
end as Fracht
from tabelle a
select RE_NR,AB_NR,
case
when Fracht=='0.00' then (select Fracht from tabelle b where Fracht !='0.00' and a.AB_NR=b.AB_NR)
else
Fracht
end as Fracht
from tabelle a