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Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?

From: Bob Badour <bbadour_at_golden.net>
Date: Tue, 18 Feb 2003 21:55:14 -0500
Message-ID: <xQC4a.50$074.5071890@mantis.golden.net>


"Steve Kass" <skass_at_drew.edu> wrote in message news:b2ufiq$g10$1_at_slb9.atl.mindspring.net...
> Excellent question. Before the present discussion, I would have
> said that the logical model of multisets is conceptually close to
> some real-world models, and certainly a good logical model for
> the "relaxed" relational model implemented by many "R"DBMS
> products, where the distinguishing attribute of rows is hidden.
>
> That said, there's also room for confusion about whether one
> conceptualizes the logical model as representing bags that
> can contain indistinguishable duplicates (imagine the bag is
> constantly being shaken, so we can't even say "the John Smith
> at the bottom of the bag"), or as representing an inventory of
> distinct entities with the additional attribute of multiplicity.
>
> These two conceptualizations of the logical model lead naturally
> to two distinct physical implementations.

Logical models need to be completely independent of physical implementation. The value of physical independence is well-known.

> The more natural one
> (in my mind) of "bags" is the more problematic, because no
> computer I've ever used has a 30 Gig "hard bag"--there will
> necessarily be an additional attribute, hidden or not, that internally
> distinguishes every item. The less natural one, using multiplicities,
> is not as convenient for some operations, such as displaying the
> contents of a multiset (with no apologies for the fact that an order
> attribute is acceptable with regard to a result set, because the
> display and the bag are two different things), but using multiplicities
> does a better job of representing the logical model, given the
> reality of actual physical computers.

What difference is there between your concept of multiplicities and an implicit count attribute tacked onto a relational logical data model?

> I really don't know if that answers your question, and I'll gladly
> admit that anything I conceptualize as a multiset can simply be
> represented logical and physically as a set, with the addition of
> the attribute of multiplicity. But just as a consistent theory of
> and notation for real numbers are useful in spite of there being
> a possible representation using only sets of (sets of ...} integers,
> I think an algebra of multisets, to use in logical models and
> to drive physical models, is useful, and it can be completely
> well-defined and consistent.

What use is it? Not only must it be useful, it must be so useful it compensates for any loss of logical identity and/or physical independence.

> SK
>
>
> Bob Badour wrote:
>
> >If the facts are truly indistinguishable, they are a single fact and we
have
> >a model based on sets. If they only represent multiplicities, one really
has
> >a relational logical data model with an implicit count attribute.
> >
> >What benefit does this implicit attribute provide?
> >
> >
> >
> >>SK
> >>
> >>
> >>
> >>>
> >>>
> >
> >
> >
> >
>
Received on Tue Feb 18 2003 - 20:55:14 CST

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