Re: foreign key constraint versus referential integrity constraint

Date: Wed, 28 Oct 2009 19:41:28 -0300
Message-ID: <4ae8c89a\$0\$26487\$9a566e8b_at_news.aliant.net>

> 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*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?

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.

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