Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!postnews.google.com!g43g2000cwa.googlegroups.com!not-for-mail From: "Alfredo Novoa" Newsgroups: comp.databases.theory Subject: Re: Translating constraints to RM Terms Date: 6 Jun 2005 01:36:34 -0700 Organization: http://groups.google.com Lines: 28 Message-ID: <1118046994.359182.212940@g43g2000cwa.googlegroups.com> References: NNTP-Posting-Host: 85.49.98.240 Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" X-Trace: posting.google.com 1118046999 31369 127.0.0.1 (6 Jun 2005 08:36:39 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Mon, 6 Jun 2005 08:36:39 +0000 (UTC) In-Reply-To: User-Agent: G2/0.2 Complaints-To: groups-abuse@google.com Injection-Info: g43g2000cwa.googlegroups.com; posting-host=85.49.98.240; posting-account=joxBkAwAAADsPEyGCMIJ7NnYIJVUS3AU Xref: dp-news.maxwell.syr.edu comp.databases.theory:31227 Kenneth Downs ha escrito: > Consider table X with columns A and B. There is a constraint that A must be > less than B. While we discuss such constraints every day, I realize I do > not know how to express such constraints in relational terms. This is one of the easiest constraints we can imagine: constraint X IsEmpty(T where B >= A); > For instance, A is actually a domain, as is B, but the constraint A < B must > in fact be part of the very definition of the domain A, and here we are > defining one domain in terms of another. No, you are confusing everything. Domains and columns are not the same. > I realized I have not seen this > disccussed in the year or so I have been a regular here. Is defining one > domain in terms of another allowed and considered trivial, no big deal? It is allowed but I would not say that it is trivial. I suggest you to read Date's Intro to Database Systems and The Third Manifesto. Although it has nothing to do with your constraint, of course. Regards