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

"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
> >>>news:51d64140.0302170330.15d2a98f_at_posting.google.com...
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>>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,

Could you provide a practical example that might help me in wrapping my mind around the notion of utility for logical models of items with multiplicities?

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