Path: newssvr20.news.prodigy.com!newsmst01.news.prodigy.com!prodigy.com!news.linkpendium.com!border1.nntp.sjc.giganews.com!nntp.giganews.com!newsfeed3.easynews.com!newsfeed1.easynews.com!easynews.com!easynews!crtntx1-snh1.gtei.net!news.gtei.net!newsfeed2.dallas1.level3.net!news.level3.com!postnews1.google.com!not-for-mail From: neo55592@hotmail.com (Neo) Newsgroups: comp.databases.theory Subject: Re: object algebra Date: 24 Feb 2004 13:08:37 -0800 Organization: http://groups.google.com Lines: 18 Message-ID: <4b45d3ad.0402241308.680a4901@posting.google.com> References: <40363638.12459115@news.wanadoo.es> <4b45d3ad.0402201644.429ee9b6@posting.google.com> <4b45d3ad.0402222141.3cd35a14@posting.google.com> <4b45d3ad.0402231444.2f77c935@posting.google.com> NNTP-Posting-Host: 67.98.160.202 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Trace: posting.google.com 1077656918 4014 127.0.0.1 (24 Feb 2004 21:08:38 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Tue, 24 Feb 2004 21:08:38 +0000 (UTC) Xref: newssvr20.news.prodigy.com comp.databases.theory:23991 > In any event, a relation is a predicate. The above definition of a relation is significantly incomplete according to RDM. See Date's Chapter 4, section 3. > In short, you've opted out of saying something specific, > and want to hand-wave about the other attributes. Sometimes it is reality doing the hand-waving. For example a person may not have any eyes, but the relation's heading has eyeColor defined. > Which is fine, given adequate subtyping - > you can model a predicate over a type, and have subtype values > in attributes of tuples in the relation. The core problem is not about subtyping but RDM's failure to handling missing data as describe by Date in Chapter 20, Missing Information.