Re: foreign key constraint versus referential integrity constraint

On Oct 28, 2:57 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> Bob Badour wrote:
> > paul c wrote:
>
> >> Tegiri Nenashi wrote:
> >> ...
>
> >>> Is view definition a constraint? IMO it's purely terminological
> >>> matter. Consider relations x and y defined by some algebraic
> >>> identities. Is adding new view z (as a function of x and y) adding a
> >>> constraint to the system?
>
> >>> Let's analyze a simpler example. Consider two real values constrained
> >>> by the equality:
>
> >>> x + y = 5
>
> >>> Is introducing a new variable z, say
>
> >>> z = x - 2y
>
> >>> a new constraint imposed onto the system? Not really, because,
> >>> variable z is redundant and can be eliminated, and it doesn't affect
> >>> the formal property of the system of being under constrained.
>
> >> That is a form of argument that I've seen quite often regarding
> >> various RM questions, not just this one.  I'd have no problem with it
> >> were it not called an "example".  Since it is about arithmetic, it's
> >> at best a mere analogy to relations and we need to decide whether the
> >> analogy should apply.
>
> > Ahem.
>
> > x + y = 5 is a relation. z = x - 2y is a relation. They are linear
> > polynomial functions, and all functions are relations.
>
> > x*x + y*y + z*z - r*r = 0 is also a relation. It is a relation
> > describing a sphere of radius r centered at the origin. It is also a
> > polynomial. While it is not a function, it is a relation.
>
> >> To try to answer that I would ask when do we ever record "extensions"
> >> of arithmetic equations?
>
> > Whenever anyone writes the word "let":
>
> > Let u = x-3, v=y+2, w=z-1...
>
> >> In other words, just because we have abstract operations for both
> >> numbers and relations doesn't mean one should mimic the other.  If
> >> that's so, maybe somebody else can put it better.
>
> > Whether involving numbers or no numbers, a relation is a relation. What
> > we can do with relations doesn't change because some of them involve
> > numbers and some of them do not.
>
> That's very good, accurate up to a point and no argument except for a
> couple of things i) when he mentioned "variable" Tegiri didn't make it
> clear whether he was talking about one of Codd's non-binary relations,

Ok, equality relation in u = x-3 certainly contributed to the confusion. I meant to bring in algebraic analogy without any reference to relations. It was just an algebra of real numbers (aka real number field). The idea was that the topic of [linear] constraints over real number field is well understood (so it can be viewed as a role model for development in database field).

Certainly, in database field we have different algebraic axioms (they are somewhat similar to boolean algebra!), but the other concepts stay the same (variables, constants, operations, and equations). The objects of the algebra are relations and not numbers, of course -- yet another source of confusion because relations structured into tables might have numbers in them! Received on Thu Oct 29 2009 - 01:10:57 CET