Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!newsfeed.icl.net!newsfeed.fjserv.net!news-lond.gip.net!news.gsl.net!gip.net!news.tele.dk!news.tele.dk!small.news.tele.dk!uninett.no!ntnu.no!not-for-mail From: Jon Heggland Newsgroups: comp.databases.theory Subject: Re: No exceptions? Date: Sat, 01 Jul 2006 00:08:14 +0200 Organization: Norwegian university of science and technology Lines: 27 Message-ID: References: <8gWog.1328$u11.1035@tornado.ohiordc.rr.com> <59_og.3924$pu3.92281@ursa-nb00s0.nbnet.nb.ca> <%C3pg.319$PE1.172@newsread2.news.pas.earthlink.net> NNTP-Posting-Host: coleburn.idi.ntnu.no Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Trace: orkan.itea.ntnu.no 1151705416 15224 129.241.111.99 (30 Jun 2006 22:10:16 GMT) X-Complaints-To: usenet@itea.ntnu.no NNTP-Posting-Date: Fri, 30 Jun 2006 22:10:16 +0000 (UTC) User-Agent: Thunderbird 1.5.0.4 (Windows/20060516) In-Reply-To: Xref: dp-news.maxwell.syr.edu comp.databases.theory:42730 J M Davitt wrote: > Jon Heggland wrote: >> There is some confusion here on both parts, I think. Any relvar can >> have an empty key, regardless of the number of attributes in the >> relvar. It follows that such a relvar can have no other keys. A >> relvar has a set of (candidate, if you will, but I consider that >> term meaningless) keys, in general, but this set cannot be empty >> ---there is always at least one key. > > I understand your point about candidate keys; I believe the term > is used to distinguish the irreducible set of attributes that hold > unique values from the sets of values that comprise superkeys. In my experience, it is used to distinguish "primary" keys from the rest. No matter. > The rest of your post confuses me. I understand that the last > sentence refers to a set of keys, but I'm not sure how it is that > there is only one key in that set of keys if that key's set of > attributes is empty. It seems to me that every attribute would be > a key and every combination of attributes would be a superkey. I > don't see how an empty attribute key precludes other keys. "Every attribute" would also be a superkey (speaking loosely). The empty set is a subset of every set. -- Jon