Re: foreign key constraint versus referential integrity constraint

From: paul c <>
Date: Wed, 28 Oct 2009 22:57:15 GMT
Message-ID: <f%3Gm.50644$PH1.37179_at_edtnps82>

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, I presume traditional math philosophy would have to have some recasting for that (don't ask me how!) ii) even if I'm wrong about i), Codd's relations are slightly different in usage both because they use different operators than the arithmetic ones and because we can 'bend' the relational ops to produce a certainty from an uncertainty, notably when we use union to 'insert' to a relation - this seems quite different to me from what arithmetic allows. Received on Wed Oct 28 2009 - 23:57:15 CET

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