Thanks for clarifying that, too bad, I guess I'll look for another way to do it, either a Trigger like you said, or maybe a two field child table, since this is for a very specific situation, thanks again.

>The UNIQUE constrait differs from the PRIMARY KEY constraint in that you can have NULLs - but only one!
>
>You can always implement a trigger that tests for uniqueness and ignores NULLs
>
>-Mike