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

From: D Guntermann <guntermann_at_hotmail.com>
Date: Tue, 18 Feb 2003 23:17:09 GMT
Message-ID: <HAJ20I.Lq0_at_news.boeing.com>

"Steve Kass" <skass_at_drew.edu> wrote in message news:b2u8ht\$9i0\$1_at_slb2.atl.mindspring.net...
>
>
> Bob Badour wrote:
>
> >"Steve Kass" <skass_at_drew.edu> wrote in message
> >news:b2rqnp\$1tj\$1_at_slb2.atl.mindspring.net...
> >
> >
> >>Mikito Harakiri wrote:
> >>
> >>
> >>
> >>>"Paul" <pbrazier_at_cosmos-uk.co.uk> wrote in message
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>>OK suppose I have an employee "relation" which is a multiset.
> >>>>I have two employees called John Smith, in the same dept on the same
> >>>>salary.
> >>>>So my multi-relation contains two outwardly identical tuples:
> >>>>("John Smith", 10, 20000)
> >>>>("John Smith", 10, 20000)
> >>>>
> >>>>Now one of the John Smiths has a sex-change and becomes Jane Smith.
> >>>>
> >>>>How does the user update only one of the rows?
> >>>>Surely it's impossible because the two rows are only distinguished
> >>>>internally?
> >>>>
> >>>>
> >>>>
> >>I think you are confusing your employees with your model.
> >>If we have only the two employees named John Smith, in
> >>department 10 with salary 20000, and we are storing this
> >>information in a multiset, we have only _one_ fact, not two:
> >>"There are two John Smiths in department 10 with salary
> >>20000". Two employees, but only one fact.
> >>
> >>If we wish to change our data so that it now represents two
> >>facts:
> >>"There is one John Smith in department 10 with salary 20000".
> >>"There is one Jane Smith in department 10 with salary 20000".
> >>
> >>we do so without any difficulty. There are many ways to
> >>devise a physical representation of our multiset, and how we
> >>transform it depends what the representation is.
> >>
> >>
> >
> >Exactly. Multisets have little utility for logical models, because one
must
> >resort to physical locations or addresses to make any use of the
duplicates.
> >
> >
> >
> I'm afraid I disagree. Multisets have plenty of utility for logical
models
> of items with multiplicities.

Steve,

I'm afraid I'm having problems understanding how one can even have some sense of determinancy of what constitutes a multiset in contrast to a set without some implicit logical mapping to identity. In the mind's eye, the very basis for contrasting a multiset from a set, or vice versa, is dependent on our very notion of identity.

For example, if I see {1,1,1,1,1}, I would have a tendency to describe it as a collection of integer 1 values with a cardinality of five. In the process of synthesizing my description, I find that I implicitly assign cardinality to each member even though set theory would reduce this to {1}. Thus, I can distinguish between the two collections.

What am I missing?

I guess the root of my confusion lies in the fact that I don't see how we relate to anything in the real world without trying to apply some notion of identity in a logical sense.

As soon as you expect multisets to model
> something
> else, you're not going to have much luck. It sounds like you are
expecting
> the multiset to model the individuality of "the duplicates", and if you
> are, then
> you are thinking of the duplicates as separate facts, and
> distinguishable. Multisets
> don't model the "individual duplicates" as individual entities. If you
> want to do that,
> you need to use a set model and include a distinguishing attribute with
> the entity.
>
> SK
>
> >
> >
>
Received on Wed Feb 19 2003 - 00:17:09 CET

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