I see no need for a relationship between codes. Plus if that rule enforces referential integrity, you can't use values that are not in the zip table.
The zip table should simply be a lookup table that your zip code combo is bound to (not to be confused with lookup field). You store the zip autonumber as an fk (foreign field) in the registration table. Your query links reg table zip fk (a long) to zip table pk (primary key) and by pulling in the zip table fields you need, you get state name, abbreviation or whatever else you have in there and need.
Last edited by Micron; 05-04-2023 at 09:38 AM.
Reason: clarification
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.