Path: news.easynews.com!newsfeed1.easynews.com!easynews.com!easynews!newsfeed.news2me.com!canoe.uoregon.edu!logbridge.uoregon.edu!newsfeed.stanford.edu!postnews1.google.com!not-for-mail
From: 71062.1056@compuserve.com (--CELKO--)
Newsgroups: comp.databases.theory
Subject: Re: Domains as relations
Date: 9 Oct 2002 17:11:34 -0700
Organization: http://groups.google.com/
Lines: 21
Message-ID: <c0d87ec0.0210091611.1f77de90@posting.google.com>
References: <51d64140.0210080104.f79cbee@posting.google.com> <8kBo9.173$0I3.13779@petpeeve.ziplink.net>
NNTP-Posting-Host: 63.52.170.201
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-Trace: posting.google.com 1034208694 25962 127.0.0.1 (10 Oct 2002 00:11:34 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: 10 Oct 2002 00:11:34 GMT
Xref: newsfeed1.easynews.com comp.databases.theory:22900
X-Received-Date: Wed, 09 Oct 2002 17:11:30 MST (news.easynews.com)

>> ... there are two ways to define a relation:  by list or by rule...
 if there is an accepted mathematical term for what I'm calling "rule
based relations". <<

We usually talk about definitions as "intensional" and "extensional";
you give a list of allowed values or you give a characteristic
function which can test any candidate value for
membership/non-membership.

Then you get in "Godel-land" and have all kinds of questions:

1) Can I use an infinite list?  How do I know a value is or is not in
that list?

2) What if my characteristic function shows only membership? Can I
assume all the rejections are not members?

3) What if my characteristic function shows only non-membership?  Can
I assume the rejections are all members?

etc.