Re: The Practical Benefits of the Relational Model

"D Guntermann" <guntermann_at_hotmail.com> wrote in message news:H5301J.6B3_at_news.boeing.com...
[snip]
> I could be wrong, but I believe I read this in Elmasri and Navathe's
> textbook, "Fundementals of Database Systems, 3rd Edition. I will
> double-check to verify.

Thanks for the pointer, sounds like it is worth a read.

> If I understand you correctly, you seem to advocate the same approach. The
> major difference being that your arguments stem from discussions concerning
> Date's principle of Orthoganal Design.
>
> >This is particually interesting in the context
> > of relvar projection.
> >
> Please explain.

The idea being (I think) that any two projections that have the same predicate should also have the same value.

So if we have
P (Part PART_#, Part_name STRING)
S (Supp SUPPLIER_#, Part PART_#)

Then

    SELECT Part FROM S
should equal

    SELECT Part FROM P

as they have the same predicate.

In otherwords we should only give the PART_# attributes in S & P the same name if every supplier supplies all parts (ie.. iff S.Part -> P.Part and P.Part -> S.Part)
If not, then the Part in S should be called something like 'Supplied_Part', because it is not holding the same information as P.Part.

Regards
Paul Vernon
Business Intelligence, IBM Global Services Received on Thu Nov 07 2002 - 19:14:01 CET