>>>Could they be FK to some tables?
>>
>>The meaning of FKs is NOT TO HAVE orphan records :-)
>
>What if you don't necessary require an entry for the ID? E.g. you may have a Key, then it should be in the child table, but you also allow to not have a relation.
>
>In our case (though VFP native tables) we used NULL in FK for such cases.

That broke all rules for data integrity.
The child table always must have corresponding record in the parent one, the opposite is not necessary.
Can you give me an example?