Path: news.cambrium.nl!textnews.cambrium.nl!feeder3.cambriumusenet.nl!feed.tweaknews.nl!postnews.google.com!m1g2000vbh.googlegroups.com!not-for-mail From: Erwin Newsgroups: comp.databases.theory Subject: Re: The Relational Model & Queries That Naturally Return Duplicate Rows Date: Sun, 10 Oct 2010 02:35:44 -0700 (PDT) Organization: http://groups.google.com Lines: 48 Message-ID: References: <0d9300b0-88be-40f3-ae94-97d1beac56dd@j18g2000yqd.googlegroups.com> <03efca15-4020-4ea2-b0c3-cfb8723e713f@l20g2000yqm.googlegroups.com> <746c7840-b5cb-4a16-98ff-aa66c3def471@j25g2000yqa.googlegroups.com> <72885253-9078-4a7d-a7e6-256ca1af6357@j25g2000yqa.googlegroups.com> <19f86d37-9d8c-40a4-b9f5-60bdb706b81b@t3g2000vbb.googlegroups.com> NNTP-Posting-Host: 77.109.100.36 Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable X-Trace: posting.google.com 1286703344 29552 127.0.0.1 (10 Oct 2010 09:35:44 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Sun, 10 Oct 2010 09:35:44 +0000 (UTC) Complaints-To: groups-abuse@google.com Injection-Info: m1g2000vbh.googlegroups.com; posting-host=77.109.100.36; posting-account=-nQufgoAAABsreOCZNqo2Uyh8O-fYVPT User-Agent: G2/1.0 X-HTTP-UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727),gzip(gfe) Xref: news.cambrium.nl On 10 okt, 04:56, "Joe Thurbon" wrote: > > In the context of recent discussion on the ttm mailing list regarding =A0 > ORDER BY, it's not clear to me that correct answer is indeed obvious. (In= =A0 > fact, given my track record on cdt, I'm not sure that I am thinking of th= e =A0 > correct answer). > > With that caveat in mind, given that, with the OP's database, the query > > SELECT FIRST_NAME from USERS order by FIRST_NAME > > presents the user with something that is not a relation, it seems strange= =A0 > to require that > > SELECT LAST_NAME from USERS > > should. (Apart from that projection is a relational operator, and order b= y =A0 > isn't, but that seems to me at least to be slightly subtle). > > Cheers, > Joe- > > - Tekst uit oorspronkelijk bericht weergeven - "slightly subtle" ?????? You mean as in "that difference could equally well not exist at all" ????? In functional style, "SELECT FIRST_NAME from USERS order by FIRST_NAME" is something like ORDER_BY(SELECT(USERS,{FIRST_NAME})), and "SELECT FIRST_NAME from USERS" is just SELECT(USERS, {FIRST_NAME}). From that, it should be indisputably clear that the type of both expressions are different. The former returns something which conveys order, and thus cannot be a relation, whereas the latter, insofar as we understand SELECT to mean "relational projection", returns a relation, which does not convey order, and cannot possibly contain duplicate tuples. If one wants SELECT to mean something different than relational projection, then I refer to "Logic and Databases", chapter 12. There, "bag algebra" is investigated (and its inferiority to relational algebra exposed).