Hi Jenny!

This is a simple one - at least as long as speed is not an issue:

SELECT Code FROM Supro, Person
WHERE (person.postal1 BETWEEN Supro.P1 AND Supro.P2 or
(person.postal1 IS NULL and
person.postal2 BETWEEN Supro.P1 AND Supro.P2))
AND person.personid = 6789

though I don't quite like that way of joining tables and would advice you
to at least try

SELECT Code FROM Person
WHERE EXISTS(SELECT 1 FROM Supro
WHERE person.postal1 BETWEEN Supro.P1 AND Supro.P2 or
(person.postal1 IS NULL and
person.postal2 BETWEEN Supro.P1 AND Supro.P2))
AND person.personid = 6789

(if code is from the table Person)

or (if code is from the table Supro - I fear this one could be slow)

SELECT Code FROM Supro
WHERE EXISTS(SELECT 1 FROM Person
WHERE (person.postal1 BETWEEN Supro.P1 AND Supro.P2 or
(person.postal1 IS NULL and
person.postal2 BETWEEN Supro.P1 AND Supro.P2))
AND person.personid = 6789)

HTH,
Set