Re: The Fact of relational algebra (was Re: Clean Object Class Design -- What is it?)

"Sean Case" <gsc_at_zip.com.au> wrote in message news:gsc-2DAF49.22140205102001_at_nostril.pacific.net.au...
> In article <1002196323.271647_at_kang.qonos>,
> "Daniel Poon" <spam_at_spam.com> wrote:
>
> > *** relation : (logic, set theory)
> > a correspondence between two sets (say A, B) represented
by
> > a set of ordered pairs, each containing one element from
> > A and one from B.
>
> > Implying you have to normalise everything into binary relations before
it
> > looks anything like the above definition.
>
> But the n-ary relations used in database design are an obvious
> generalisation of the above. What would you call them?

You are right. In the maths that I did, binary relations were 'used' the most, becuase they can show the sturture between things. A binary relation can show a partial ordering (us computer guys call a tree), or a full ordering (e.g. 1 < 2 < 3 < 4 < 5). i.e.
  1 < 2
is true
  2 < 1
is false

A sub class of relations have a special name. They are called functions. for example if you have a relation called 'f', then if

   x f y
is true, you can write
  y = f(x)
And we use functions all the time in maths.

Daniel Received on Fri Oct 05 2001 - 15:32:13 CEST