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Home -> Community -> Usenet -> comp.databases.theory -> Re: Grammatical Inconsistencies
According to Elmasri & Navathe...
In Relational Theory, the term JOIN implies a "join condition" that prevents a Cartesian product. So, there are two terms used, JOIN and CARTESIAN PRODUCT. They are mutually exclusive. You might specify
on dept_nbr
or, you might, for some unknown reason specify
employees CARTESIAN PRODUCT departments
Of course, thge above would be expressed in relational algebra, which can't be reproduced in text. But, you can see that I have specified two completely different operation. Note that you can't specify
employees JOIN departments
as this is incorrect.
Now, we understand in a certain way what we perceive to be really going on, and that's where the confusion comes in. In SQL, we know that when you SELECT from > 1 table, all tuples are "joined" so that there is one new tuple for each possible combination- and the result is a Cartesian product. But the "joining" is just our mental interpretation of what we imagine to be happening. If you want to truly, relationally JOIN the two tables, you need a WHERE clause to specify the JOIN criteria.
"Dawn M. Wolthuis" <dwolt_at_tincat-group.com> wrote in message
news:c697n2$fis$1_at_news.netins.net...
> OK -- I thought all of these operations were on relations and returning
> relations.
>
> Thanks for enlightening me EVEN THOUGH you said you would filter me out.
> Are you willing to bother showing me an example of the use of a join
within
> the relational theory framework where it is not the same as the
> cross-product -- or pointing me to some such example? I suppose I'm
pushing
> my luck, eh?
> --dawn
>
> Timothy J. Bruce" <uniblab_at_hotmail.com> wrote in message
> news:b4Vhc.1344$17.159553_at_news1.epix.net...
> > [I'm going to hate myself in the morning for this...]
> >
> > The cartesian product of any number of sets defines a RELATION.
> > The intersection, union, and symetric difference of any number of sets
> > defines a SET.
> >
> > Patrick Suppes: ``Introduction to Logic'', Van Norstrand Company, Inc
> > (August 1968)
> > Ralph P. Grimaldi: ``Discrete and Combinatorial Mathematics'',
> > Reading-Mass.: Addison-Wesley (1985)
> > Larry J. Goldstein, David I. Schneider, Martha J. Siegel: ``Finite
> > Mathematics And Its Applications'', Prentice-Hall, Inc (1995)
> > Kolman, Bubsy, Ross: ``Discrete Mathematical Structures'', Prentice-Hall
> > (2000)
> > Donald E. Knuth: ``The Art of Computer Programming: Volume 1:
Fundamental
> > Algorithms (third edition)'', Addison-Wesley (1997)
> > Donald E. Knuth: ``The Art of Computer Programming: Volume 3: Sorting
and
> > Searching (second edition)'', Addison-Wesley (1998)
> >
> > But what would Knuth know since he isn't a ``relational guy'',
> > Timothy J. Bruce
> > uniblab_at_hotmail.com
> > </RANT>
> >
> >
>
>
Received on Thu Apr 22 2004 - 15:21:32 CDT