Re: Constraints and Functional Dependencies

mAsterdam wrote:
> ...
> Ornamental? I am a non-native speaker of English - maybe a
> native speaker (you are, aren't you?) can put into less words
> than this what I mean with it.
>
> How would you say: "The stuff referenced is not a tuple in S,
> but a subset of S".
> ...

I wouldn't, at least I hope even in a weak moment, I would remember to not try to say anything about what the "stuff" is not. "NOT" is important because it allows us to apply boolean logic to relations but it is harder to talk about negation in precise language because what something is not increases what we try to talk about sometimes by orders of magnitude. (I suppose that is also one reason why we usually choose not to record logical complements which most often include tuples that constraints would not allow in the "positive" relations.)

>> In ordinary English, one relation references another. 

Relations, or if you like, projections of relations.

> What do you call the stuff being referenced?

Relations, or as above.

> ...
> .... which had an admitted flaw in the definiendum
> which I am trying to iron out with a rephrase suggestion.
> What is your current reading of the definition? What does it define?
> ...

I thought Marshall had retracted "foreign key" and meant it to be replaced with "referential integrity". My only problem with the latter is that I haven't been able to find a quote that I can trust where Codd indicated whether "referential integrity" involves a "candidate key".

> ...
> I like this property: If b is a ck of S, every R-tuple has an associated
> S-tuple - but admit to having no particular problem,
> solved by restricting b to a ck of S.

Heh, I usually find it impossible to try to argue against people's likes or dislikes. I would just say that problems of understanding increase when the vocabulary is increased based on them. Rather than trying to qualify a formal definition by adding a notion of "association" in some vague boilerplate, my liking would be to drop some other term in exchange. (If the motive is to sugar-coat the formal definitions for an layman's illumination, I'd vote for teaching him the formal definition instead. If he's disinterested, I'd say forget the whole idea, let him decide on his own if he wants to use the application or not.)

cheers,
p Received on Sun Feb 25 2007 - 18:02:59 CET