"Helen Borrie" wrote:

> In Firebird, since Fb 1.5, it has been possible to create a unique *index* that contains a nullable key that could be null for
> only one record in the table. (If you are interested, the rules are in the Fb 1.5 release notes.) However, Firebird does not
> allow a unique CONSTRAINT that contains nullable columns, since the purpose of a unique constraint is to provide an alternative
> key for foreign key references.

Firebird _does_ allow Unique Constraint on nullable columns.
(But of course it does not allow PK Constraint).

Ivan P.