Re: foreign key constraint versus referential integrity constraint
Date: Mon, 26 Oct 2009 09:00:14 GMT
Message-ID: <yydFm.50294$PH1.7159_at_edtnps82>
Mr. Scott wrote:
> "Marshall" <marshall.spight_at_gmail.com> wrote in message
> news:386975f9-472c-4184-8661-5c3d1e2f7621_at_r24g2000prf.googlegroups.com...
>> On Oct 24, 10:53 am, Keith H Duggar <dug..._at_alum.mit.edu> wrote: >>> Anyhow, the question here is not one of our imagination but rather >>> simply this: if it makes sense for the RM to support constraints >>> on relational /values/ (taken on by variables) why does it not >>> make sense to support constraints on relational /expressions/? >>> That is a question of general principle not specific design. >> This question, it seems to me, is clear and to the point. >> And I would answer it by saying that we shouldn't really >> even make the distinction! (At least not formally.)
>
> I think we should make the distinction, and formally.
>
> (p /\ q) -> r is not the same as (p -> r) /\ (q -> r)
> but (p \/ q) -> r is the same as (p -> r) \/ (q -> r)
>
> A view consisting of a natural join, for example, represents a set of
> conjunctions. Each row of the join represents a conjunction of
> propositions, one for each operand. A constraint defined on a join would be
> of the form (p /\ q) -> r. That is definitely not the same as constraints
> defined on one or more tables, which would take the form (p \/ q) -> r.
>
...
I guess the attitude, interpretation if you like, that relational ops implement logic leads to that but another attitude is that they merely apply logic to obtain relations that consist of simple propositions. I believe most people happily accept the latter interpretation when looking at a relation value that has been obtained by a language devices such as insert or assignment where the definition is based on union. The 'OR' disappears. I think there is a big difference between the implementation and the application of logic. Another question is what happens to the join's conjunction when we project, does it survive or not depending on which attributes we choose? Received on Mon Oct 26 2009 - 10:00:14 CET