> Hallo Hugo,
>
> > When should I use a Unique Key instead of a Index?
> >
> > I know that a Unique Key does not allow duplicated values like a Primary
> > Key, but what more?
>
> That, and it creates an index automatically. Also, if you want a column to be referenced by a foreign key, it has to be either the PRIMARY KEY or UNIQUE.

Notice that since Firebird 1.5, a unique key constraint does allow multiple null instances in the column.

http://www.firebirdsql.org/file/documentation/reference_manuals/reference_material/html/langrefupd25-ddl-table.html#langrefupd25-ct-unique-keys


Cheers,
Paul Vinkenoog