--- In ib-support@yahoogroups.com, "Carsten Schaefer"
<ca_schaefer@g...> wrote:

> But why can i insert a foreign key for this column without any
> exception.
> The field id_mitarbeiter is primary key for t_mitarbeiter and there

is

> no null value allowed.

Carsten, I can't recall exactly when (what was FB build number, but
definetely before release of FB1) this feature was included, but FK
can contain not only values which match PK of referenced table but
nulls too. Perhaps Martijn can remember more exactly, he was among
those who wanted this feature. Personally I prefer old, more
restrictive behaviour, but I was among minority and feature was
approved by developers and implemented.

Best regards, Alexander.