Re: object algebra
Date: Fri, 27 Feb 2004 21:18:21 GMT
Message-ID: <xuO%b.18054$P_4.12318_at_newssvr31.news.prodigy.com>
"Neo" <neo55592_at_hotmail.com> wrote in message
news:4b45d3ad.0402261307.1315965d_at_posting.google.com...
> > Also now the domain/type EYE_COLOR has tuples "Not Applicable" or
> > "Unknown". Doesn't this violate basic set theory?
>
> As Marshall pointed out, Color = {Red, green, blue, "Not Applicable",
> "Unknown"} is a valid set. I stated incorrectly, however the effect
> remains similar: weakens realtionship between set and its elements and
> hinders closure.
Closure doesn't have to do with the set, of course, but with the type - you're talking about operations over relations. I don't think the set above has a problem with closure, though. Joins, projections, etc. which reference an EYE_COLOR attribute can do so meaningfully and without the ambiguities of nulls. Given proper type definition facilities, you can do whatever you like with the NOT_APPLICABLE value. Received on Fri Feb 27 2004 - 22:18:21 CET