I have a table that contains (among others) the following columns:

item ID, BIGINT, not null
supplier ID, BIGINT, not null
supplier code, varchar.

the last column is the code used by the supplier for a given item. It
can be null if the user does not know it, or if he does not care.
Obviously the supplier code must be unique for each supplier, so I tried
to add an UNIQUE constraint to the last two columns, but it did not
work. If I have many rows without supplier code I have the same supplier
id in each row, and the supplier code is NULL. Firebird treats these
rows as duplicate and I cannot create the constraint.

How can I force uniqueness for supplier id and supplier code, ignoring
null supplier codes?

Thanks in advance.

Fulvio Senore