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

x wrote:
>>consider the statements:
>>1) Pete is 29 years old.
>>2) Everyone's age is under 60.
>>
>>In a relational database the first would be data and the second a
>>constraint. In Prolog would they both be data?

> 
> Nothing would stop you to say John is 84 years old.
> It just follows that John isn't "everyone".

In a DBMS, the constraint *would* stop you saying John is 84, that's the whole point of it. I presume that in Prolog it works in a similar way (I know, dangerous to talk about things I don't really know!). Surely Prolog wouldn't let you store two mutually contradictory statements?

> There is no need to provide more references. > I am aware of that.

Sorry, I'm not meaning to be patronising, I just think it's interesting to look at different ways to express it.

> The key word here is "complete".
> What is "complete" for one, it isn't "complete" for everyone.

Agreed, we have to be very careful about what exactly we mean by "complete".

>>I guess anything can be regarded as a theory in the right context.
>>In terms of Godel's Completeness Theorem you have:
>>
>>1) first-order logic itself (corresponding to the DBMS)
>>2) various theories we talk about using logic (corresponding to databases)
>>3) various models of the theories (the real-world interpretations of the
>>databases)
>>
>>1) is the meta-language, 2) is the syntax and 3) is the semantics.
>>Godel's Completeness Theorem says: suppose you talk about a theory with
>>first-order logic. Then if something is true in every possible semantics
>>for that model, it is provable using syntax alone.

> 
> I'm not sure about this.
> There must be exactly *ONE* real-world "interpretation" of the database.

Why must there?
consider the database with one tuple like this: (1,2) There could be many real-world interpretations of this database surely? It's not inconceivable that two people who've never met have identical databases with totally different interpretations.

>> > I'm not sure a database is a finite set of axioms.
>>
>>Why not? A databases is just a finite set of tuples from which you
>>derive other truths.

> 
> It is not. It is a one-to-one corespondence with a piece of the
> "real-world".

OK maybe I shouldn't have used the world "just". But why can't a database be both? From a proof-theoretic point of view it's what I said.  From a model-theoretic point of view it's what you said. The two aren't mutally exclusive.

Paul. Received on Fri Jun 04 2004 - 19:03:02 CEST