Subject Re: [firebird-support] Primary Key x Unique Key
Author Paul Vinkenoog
> 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.

Paul Vinkenoog