Ok I get it, I'm always a bit slow off the mark..

Jon


>Jon,
>
>I think he's wanting to check that the value is either null or a single other value. So the set of possible values is something like: ( null, 'fred' )
>
>>So you want to have a field that allows nulls but all non null values should be unique? Is this correct?
>>
>>If so then you cannot use a primary or unique constraint, as David suggests add a check constraint on the field to ensure its uniqueness. You could also, if you want, do this validation through a trigger but the Check constraint would be the best way to do this.