PS: Das B.Adr ist nicht NULL sondern ''
OK, und so (ohne reale Daten kann man das hier nicht testen)?
Code:
(select SUM(B.Brutto) as Summe from BEL B
where
(B.Art = 'BLUBB' or B.Art = 'BLA') and
((B.Adr > '' and B.Adr = ADR.Adr) or
(B.Adr = '' and B.KDNR = ADR.Adr))