Re: In an RDBMS, what does "Data" mean?

In message <FZJxc.388$Pt.151_at_newssvr19.news.prodigy.com>, Eric Kaun <ekaun_at_yahoo.com> writes
>"Anthony W. Youngman" <wol_at_thewolery.demon.co.uk> wrote in message
>news:UXeCeOK16OxAFwRR_at_thewolery.demon.co.uk...
>> >So what mathematical axioms do you know of that "map to reality"? I
>didn't
>> >realize that was the fundamental aspect of an axiom's value. And if it
>is,
>> >then again, what data axioms do you propose as a good start? They needn't
>be
>> >formal, but have to have more meaning than "data comes in tuples".
>>
>> e=mc^2 ?
>>
>> Yep. I know it's bl**dy difficult. But if you're not prepared to attempt
>> it, then you're admitting your theory is irrelevant to the real world
>> (and cannot be used to solve real-world problems).
>
>That's silly. The following have all been used to solve real-world problems:
>- Pick
>- SQL
>- Relational (assuming Dataphor has at least one real-world solution in
>place somewhere)
>- Flat files
>- XML
>
>I don't know the "axioms" of the flat-file solution, and don't think that
>Unix's "everything is a file" is really an axiom. What they're saying is
>that we have a useful model that treats all data as files.
>
>Besides, who is attempting it? What attempt has MV made? I don't understand
>what you're looking for here - do you want science, math, or neither? In
>whatever category you want, what does Pick/MV offer? You seem unwilling to
>pick up your own gauntlet.

What I'm saying is that maths is great at building a model. But without science you can't say that any model is useful. Without science, a model is just an intellectual exercise of no value to the real world.
>
>> Let's take the evolution of that theory I keep on throwing out as an
>> example.
>>
>> Copernicus : orbit == circle
>> Kepler : obit == ellipse
>> Newton : F=ma; E=1/2mv^2 where m is constant
>> Einstein : e=mc^2
>>
>> Each change may only subtly modify the previous axioms, but the result
>> is theory/model that is a closer fit to reality.
>
>I don't think the above are axioms in the mathematical sense, though I could
>be wrong.

Yes they are. Copernicus ASSUMED that the planets went in circles, and then he used logic on top or that. Therefore, "orbit == circle" is an axiom.

Kepler realised that "orbit == ellipse", and that explained why Copernicus' logic was so screwy.

Newton ASSUMED that m and E could neither be created nor destroyed, therefore they are axioms.

Einstein realised that m and E were interchangeable, and that explained why Newton couldn't predict the orbit of Mercury.

Basically, any assumption that underlies mathematical logic is an axiom. Copernican orbital theory is a mathematical model. Newtonian Mechanics is a mathematical model. Therefore the assumptions that underlie them must be axioms.
>
>> Going back to relational theory. Does the THEORY distinguish between a
>> "join" and a "join with a cascading delete"?
>
>Cascading deletes are useful for implementations, not part of the theory - a
>cascading delete is simply nice shorthand for an implicit multi-update (as
>advocated by Date in recent writings), and roughly corresponds to the
>usefulness of the "foreign key" concept in place of a longer-winded
>constraint definition.

In other words, the theory has no way of coping with what I call "the adjectival clause" - a table whose contents are meaningless without the existence of another table to point to. An invoice line item cannot exist without an invoice for it to belong to!

Or, in other words again, relational theory is deficient because it has no way of coping with real-world constructs that "obviously" exist.
>
>> Or a "join" and a "join with a foreign key that must exist (cannot be
>null)".
>
>In relational, all foreign keys must exist, and no attribute value can be
>null.
>
>> Because if relational theory cannot cope with that, then the Pick model
>can.
>
>"Cannot cope with that" implies that there is some objective reality that's
>presenting X, and that a model that doesn't "cope with" X is a poor match to
>reality. While I agree with the implication overall, the premise is false -
>there's no objective reality "presenting" cascading deletes or nulls. Those
>are both aspects of modeling data. There's no objective reality with which
>those correspond. At best, you're pitting Data Model A against Data Model B,
>and claiming B is lacking in attribute C, when C doesn't even enter into
>Data Model A.

But there IS objective reality. A line-item on an invoice, for example. The former has no existence outwith the latter.
>
>> And surely, a relational table who's rows are meaningful in their
>> own right MUST be different from a table who's rows are meaningless
>> without another table to relate to?
>
>"Meaningful in their own right" is rhetorical - every relation has a meaning
>(the external predicate). To turn the question on its ear, surely a Pick
>file which requires applications to enforce the correspondence between
>values in several distinct attributes MUST be different from a file whose
>attributes refer to the IDs of other files?
>
I don't get that. But I think you're making the logical blunder of expecting your logic to PREscribe the world's behaviour, rather than DEscribe it.

Please explain to me how, in the real world, an invoice line item can have an existence in the absence of the invoice to which it belongs ... because as I read you you are saying that relational theory says it can ...

Cheers,
Wol

-- 
Anthony W. Youngman - wol at thewolery dot demon dot co dot uk
HEX wondered how much he should tell the Wizards. He felt it would not be a
good idea to burden them with too much input. Hex always thought of his reports
as Lies-to-People.
The Science of Discworld : (c) Terry Pratchett 1999