Christian,

>> What I do is have a "master" key of zero, which I assign the foreign key
>> when I want to "park" the dependent rows somewhere.

C> I had thought of this too, but I thought there was a more "Firebird"
C> way to do this. If there is no other way, I will use this workaround.

as others have pointed out, you are most probably being fooled by a
tool or interface you are using to perform your tests. Having nullable
foreign keys regardless of the "on delete" setting is legal not only
in Firebird but in every decent SQL-compliant database that I know
(admittedly, there aren't many of them ;-)).

Have a look at the employee database and find the foreign key between
PROJECT and EMPLOYEE, which looks like an example of what you're after.

HTH
--
Nando Dessena
mailto:nandod@...