Re: One-To-One Relationships

On Dec 1, 4:44 am, Marshall <marshall.spi..._at_gmail.com> wrote:
> On Nov 30, 5:29 am, David BL <davi..._at_iinet.net.au> wrote:
> > On Nov 30, 5:09 pm, Marshall <marshall.spi..._at_gmail.com> wrote:
> > > On Nov 29, 10:50 pm, David BL <davi..._at_iinet.net.au> wrote:
> > > > On Nov 30, 2:54 pm, Marshall <marshall.spi..._at_gmail.com> wrote:
> > > > > On Nov 29, 9:20 pm, David BL <davi..._at_iinet.net.au> wrote:
>
> What I was saying is that entities, as a logical concept, don't
> add anything that we don't already have with propositions.

Agreed

> Suppose I say
>
> M(s)
>
> What does it mean? Nothing, without an interpretation.
> My interpretation
>
> s = Socrates
> M(x) = x is a man.
>
> The interpretation is in your head. The interpretation is a
> mapping from the formally stated concepts to words,
> ideas, thoughts, whatever is in your head.

> > Let me ask it a different way: Without any notion of entities in the
> > real world, isn't a formal proposition just a meaningless formula?
>
> I don't think so. Can't you have formal propositions about
> abstractions?
>
> There is no largest integer
> !ExAy(x>y)

Sure, but I was really only thinking about the RM.

The purpose of the RM isn't to make formal propositions about pure abstractions. In fact there is a sense in which tautologies have zero information content.

> > I understand how it is important that the formalism itself does not
> > have any conception of entities. It seems to me there is an important
> > line in the sand, with
>
> > formalism,
> > propositions/tuples,
> > predicates/relations,
> > extensions
>
> > on one side, and
>
> > informalism,
> > the real world,
> > entities,
> > intensions,
> > natural language,
> > facts
>
> > on the other.
>
> That seems like a very heterogeneous list to me.

It is the dividing line between formalism and informalism.

The negativity about entities seems to arise from their informal nature. Do we say the real world is unimportant as well?

> > > > The propositional calculus is a formalism and doesn't come with some
> > > > mapping back to the real world. I don't understand how any mapping
> > > > could be understood without any conception of entities.
>
> > > Does that mean you understand the term "entity" to be referring
> > > to the conceptual layer? I don't think that's the usual sense of
> > > the word.
>
> > I have read a number of posts from cdt talking about conceptual vs
> > logical layers but I'm afraid I still don't understand the
> > distinction.
>
> > My understanding of conceptual layer comes from Date's 3 level
> > description in chapter 2 of An Introduction to Database Systems, and
> > I'm not sure how that is relevant to this discussion. I understand it
> > as the intermediary level providing both physical and logical
> > independence. Can you elaborate?
>
> I'm probably not the best person for that. But, to my way of thinking,
> the conceptual layer is what's in your head.

That's quite different to Date, who suggests the conceptual layer should use the RM to provide a view of the data "as it really is", in contrast to external views for particular users.

I wonder whether Date wanted to expel the ER approach from the outset, because he assumes all one needs in one's head (at the formal level) is the RM.

I tend to think that the intensional definitions provide the connection between what I imagine you would call the logical and conceptual layers, not being a fan of ERM either.

> My Occam-inspired view: you see stuff and you hear stuff, and
> you get ideas in your head. You'd like to operate on these ideas
> in a formal way. You decide to use relations to do so. So you
> need to map what you think into relations. You could go though
> any number of intermediate steps to get there. But don't! Just
> write down what goes in the darn tables.

Well put.

[Quoted] Unfortunately many "software engineers" like to learn methodologies, hoping for a free lunch. Received on Sat Dec 01 2007 - 03:49:10 CET