Re: compound propositions
Date: Wed, 17 Mar 2010 03:21:09 -0700 (PDT)
On Mar 17, 4:01 am, paul c <toledobythe..._at_oohay.ac> wrote:
> I would say tuples are abstractions, also that they are not
> propositions, just partial representations.
The view which is generally promoted in db design and programming courses is to view tables as containing entities or parts of entities, which is a faulty mapping from the entity-relationship model to the relational model. Is that what you mean?
> >> If those relations were 'exclusive', I think the proposition would be
> >> something like ( 'C1' is a customer and 'C1' is not a client ) OR ( 'C1'
> >> is not a customer and 'C1' is a client ).
> > I'm sorry, I don't follow. Would you mind rephrasing, please?
> That is a proposition which satisfies an 'exclusive-or' predicate, which
> I think is the assumption you had in mind when you mentioned 'exclusive'
> in the previous message and 'prevent the conjunction' in this one. By
> giving the 'xor' proposition, I was just trying to point out that it was
> different from the original one I had given and that I hadn't made that
> assumption about disjoint values.
My apologies, it seems I jumped to conclusions.
> I think one reason people interested in db theory spend so much time on
> predicates is because of the practical problem of expressing union (and
> negation), eg., this proposition might be more clear if expressed by
> tuples with four attributes, but the storage to materialize that is
> usually impractical. So instead most if not all dbms' expect us to use
> what Codd called 'union-compatible' relations for a result that has only
> one attribute.
The reason db theory people spend so much time on predicates is because a relational database is a representer of facts. Predicates and propositions are fundamental to the whole model and theory.
I'm not sure what you mean in the rest of the paragraph, since I don't see a difference between tuples and propositions and I don't see what storage has to do with it.
> I don't object to people talking about entities instead of the actual
> 'objects' of an algebra, after all Codd talked about them. But I think
> he did that to help his exposition, not to suggest that an algebra or
> FOL could ever operate on entities (at least in his papers before 1974
> or so, not sure about later nor his book).
I think the whole DSL movement is an attempt to recreate the algebraic abilities lost in the assumption of the entity-relationship model.
> I assume my original
> question is at fault for leading to this 'exclusive' assumption because
> it was probably too fuzzy.
It's very likely that I'm somewhat fuzzy too. ;) Received on Wed Mar 17 2010 - 11:21:09 CET