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

Date: Sun, 20 Jun 2004 01:37:08 GMT

Message-ID: <40d4ea3f.9273674_at_news.wanadoo.es>

On Sat, 19 Jun 2004 09:27:18 +0100, Paul <paul_at_test.com> wrote:

>> If T is a set of axioms in a first-order language, and a statement p

*>> holds for any structure M satisfying T, then p can be formally deduced
**>> from T in some appropriately defined fashion.
**>
**>I think the problem is how do we apply this theorem to relational
**>databases. The way I see it, it fits with what I said. :)
*

For instance it is possible to prove this:

a minus (a minus b) = a intersect b

>When you say "if a relational formula is valid or not" do you mean valid

*>as in well-formed or valid as in true?
*

In true.

> If you mean true, then maybe we

*>are just saying the same thing in different ways.
*

Yes. The antecedent and the consequent are equivalent.

> But I'm seeing the

*>result as profound and you're seeing it as trivial.
*

>Thus I see Godel's completeness theorem as saying we can therefore prove

*>the statement '(forall x) q(x,20)' purely syntactically i.e. without
**>considering semantics at all.
*

Yes, it means that it is a FORMal statement because it is valid depending on the syntactical form and not on the semantics.

> The DBMS only knows syntax, not semantics.

*>Thus the DBMS can prove the statement on its own.
*

Indeed.

>Now it's meaningless to ask whether our new predicate is true or not.

It is always meaningless to ask whether a predicate is true or not :)

What are true or not are the propositions.

>What we want to know is whether any question we ask in semantic terms

*>can be answered syntactically. For example the question 'Does Alan like
**>rice pudding?'.
*

var Likes relation { a char, b char };

Likes := relation {

tuple { a 'Alan', b 'Rice Pudding' }

};

if tuple { a 'Alan' } in (Likes where b = 'Rice Pudding') { a } then ...

>But I

*>think that all first-order questions that can be answered semantically
**>will translate to a syntactic question. Which can definitely be answered
**>(by Godel's completeness theorem?)
*

Indeed.

Regards

Alfredo
Received on Sun Jun 20 2004 - 03:37:08 CEST