Re: foreign key constraint versus referential integrity constraint
Date: Sat, 24 Oct 2009 16:13:32 -0300
Message-ID: <4ae351b6$0$23749$9a566e8b_at_news.aliant.net>
Keith H Duggar wrote:
> On Oct 22, 7:34 am, Sampo Syreeni <de..._at_iki.fi> wrote:
>
>>>Thanks to everyone. I think I understand more clearly now the
>>>inherent asymmetry of foreign key constraints. But now then my
>>>question is what is the common name for the semantic constraint
>>>that "attribute RV1:X and RV2:Y are sematincally joinable"?
>>
>>Hmm. Usually we talk about constraints when they're syntactic, because
>>that's what makes it possible to enforce them automatically, simply by
>>manipulating the symbols in the database. So what you actually mean
>>is, what syntactic construct is appropriate for enforcing the
>>semantics you describe. I'd argue that that's what domains were
>>invented for.
>
> I agree that it is possible to enforce such join semantics with
> domains but I'm not sure it is entirely appropriate. I say that
> because to the greatest extent the RM is orthogonal to domains.
> To quote CJ Date "Databases In Depth" Chapter 2 Summary:
>
> "It's a very common misconception that the relational model deals
> only with rather simple types: numbers, strings, perhaps dates
> and times, and not much else. In this chapter, I've tried to show
> that this is indeed a misconception. Rather, relations can have
> attributes of /any type whatsoever/ -- the relational model
> nowhere predicates what those types must be, and in fact they
> can be as complex as we like ... In other words, the question as
> to what types are supported is orthogonal to the question of
> support for the relational model itself. Or (less precisely but
> more catchily): /types are orthogonal to tables/."
>
> To put this another way, the RM should be just as complete and
> effective for a universe with a single domain as it is for a
> universe with a rich domain set (such as you propose to handle
> my GenericID join constraint example). And placing constraints
> on /relational expressions/ seems as fundamental to the RM as
> placing constraints on /relational values/.
>
> By pushing such expression constraints off to domains, we are
> admitting that the RM itself has no support for constraining
> relational expressions and I'm not prepared to admit that
> limitation. Especially since it does provide support for
> constraining the values of relational variables.
>
> I will also note that both Codd and Date proposed RM mechanisms
> to operate the other way around ie to allow designers to force
> cross-domain operations. Codd with "Domain Check Overrides" and
> date with the THE_ operators that provide type coercion.
Domains, however, define what is or is not joinable because natural join applies the equality comparison to attributes with like names, and domains define equality comparisons. Received on Sat Oct 24 2009 - 21:13:32 CEST