Re: MV Keys

"Jon Heggland" <heggland_at_idi.ntnu.no> wrote in message news:MPG.1e77dd36ef215d5989796_at_news.ntnu.no...
> In article <N5hPf.56961$dW3.6378_at_newssvr21.news.prodigy.com>,
> brian_at_selzer-software.com says...
>> > Why? And what do you mean by "arriving at a list of widgits through the
>> > use of the combinatorial rules of the universe"?
>>
>> I think I made my point clear earlier. The universe is the set of all
>> possible values that are relevant to the discussion at hand and a set of
>> rules for combining those values to form composite values, or
>> propositions.
>> Composite values are indeed propositions. If you have values A and B and
>> group them together, then "A is associated with B" is a true statement.
>
> I disagree. A value in itself is not a fact. In the context of the RM,
> only tuples in relations are facts. The cartesian point (2,3) is not a
> fact; the numbers 2 and 3 don't become more associated with each other
> just because I refer to this point. Only when it is put into a relation
> can it become (part of) a fact, e.g. "A FooBar is located at (2,3)".
>
You're right. A value in itself is not a fact. Perhaps I should be a little more precise. A composite value is a sentence which, once stated in a database, becomes a fact. "x is 2" is a sentence; "y is 3" is a sentence. "x is 2 and y is 3" is also a sentence. The juxtaposition of 2 and 3 in the ordered pair representing a cartesian point is a sentence. Furthermore, in the RM, attribute values are facts, not just tuples, because all stated values in the RM are named. Thus a tuple in a relation R{A, B, C} is the set of propositions {A has value 3, B has value 7, C has value 2}. The tuple itself also has a truth value, "A has value 3 and B has value 7 and C has value 2." which is a proposition in conjunctive normal form.

>> >> If you define constraints on the widgit list
>> >> domain, but construct some widgit lists in a database without
>> >> referencing
>> >> that domain, then those constraints cannot be enforced for every list
>> >> of
>> >> widgits.
>> >
>> > So what? If you want domain constraints, you have to create a domain---
>> > which is then distinct from other domains. I can create a domain of
>> > integers between 42 and 5286; that does not preclude using integers
>> > outside that range elsewhere in my database.
>>
>> You're missing the point. The whole numbers between 42 and 5286 are the
>> magnitude of some class of values relevant to the discussion. Integers
>> outside of that range must belong to a different domain.
>
> Precisely. Just as lists that don't conform to your constrained widgit
> list must belong to some other domain.
>

The problem is that the widgit list domain is a set of lists of widgits, which can be constructed using the combinatorial rules. Some of the constructed lists may not violate the constraints, so the meanings overlap. This is not true of the use of integers in multiple places, because integers describe one aspect of a domain, they're not the domain.

>> >> Without First Normal Form, there's no limitation on the complexity of
>> >> an
>> >> attribute.
>> >
>> > Nor is there *with* 1NF---but that might of course depend on you
>> > definition of both "complex" and "1NF". How complex is a BLOB? An
>> > image?
>> > A fingerprint? A video? I say any of these can be used as an attribute
>> > value, and doing so doesn't affect 1NF.
>>
>> Only if they cannot be resolved into components that are part of the
>> universe of discourse. It's important to differentiate between
>> resolution
>> and transformation, at least how I'm using the terms. A composite value
>> can
>> be resolved into component parts, meaning that an association exists
>> between
>> the component parts. A possible representation of a value can be
>> transformed into another possible representation, but the value does not
>> change. For example, a point can be represented using cartesian
>> coordinates
>> or polar coordinates, but it's still the same point: no information is
>> lost.
>
> I know, but I don't see the relevance. But since I don't accept your
> claim that a composite value constitutes a fact, it is pointless to
> belabour this point.
> --
> Jon
Received on Wed Mar 08 2006 - 11:27:02 CET