Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!newsfeed.icl.net!feed.news.tiscali.de!fu-berlin.de!uni-berlin.de!not-for-mail From: Kenneth Downs Newsgroups: comp.databases.theory Subject: Re: 4 the FAQ: Are Commercial DBMS Truly Relational? Date: Sun, 10 Oct 2004 10:35:38 -0400 Lines: 30 Message-ID: References: <2srmk1F1njk29U1@uni-berlin.de> <9n2ad.153703$wV.90666@attbi_s54> Reply-To: sameas@email.address Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7Bit X-Trace: news.uni-berlin.de SFU+b3gLYDajMento2UKKQNcYuVEhjPsR+loYjnnqW3TcOwfke X-Orig-Path: mercury.downsfam.net!nobody User-Agent: KNode/0.7.7 Xref: dp-news.maxwell.syr.edu comp.databases.theory:26729 Marshall Spight wrote: > "Christopher Browne" wrote in message > news:2srmk1F1njk29U1@uni-berlin.de... >> >> If you cut columns off of the result set, it is possible for the >> result set to, in fact, not be a "set", but rather a non-unique "bag" >> of tuples. >> [...] >> That is NOT going to be a "set" or a "relation" if some customer made >> multiple purchases between those dates. >> >> The problem that this expresses is that the relational algebra does >> not satisfy the property of closure. > > That's an odd viewpoint. > > The math books I've read have made the point that when you > are talking about sets, then {2, 2} is the same set as {2}. > So if you have duplicates after a project, you just throw > them away; they "don't count" so to speak. > Therefore, wouldn't the implementation of SQL have some sort of implied DISTINCT on every SELECT statement? -- Kenneth Downs Use first initial plus last name at last name plus literal "fam.net" to email me