Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!wn14feed!worldnet.att.net!attbi_s53.POSTED!53ab2750!not-for-mail From: "Marshall Spight" Newsgroups: comp.databases.theory References: <2srmk1F1njk29U1@uni-berlin.de> <9n2ad.153703$wV.90666@attbi_s54> Subject: Re: 4 the FAQ: Are Commercial DBMS Truly Relational? Lines: 37 X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 6.00.2800.1158 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165 Message-ID: NNTP-Posting-Host: 24.5.186.86 X-Complaints-To: abuse@comcast.net X-Trace: attbi_s53 1097421808 24.5.186.86 (Sun, 10 Oct 2004 15:23:28 GMT) NNTP-Posting-Date: Sun, 10 Oct 2004 15:23:28 GMT Organization: Comcast Online Date: Sun, 10 Oct 2004 15:23:28 GMT Xref: dp-news.maxwell.syr.edu comp.databases.theory:26731 "Kenneth Downs" wrote in message news:pbhbkc.5d7.ln@mercury.downsfam.net... > 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? We've switched from talking about the relational algebra to talking about SQL. Yes, the relational algebra has an implied "distinct" on the result of every operation. No, SQL doesn't. This is one of the ways SQL isn't relational. Marshall