Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!uio.no!ntnu.no!not-for-mail From: Jon Heggland Newsgroups: comp.databases.theory Subject: Re: MV Keys Date: Fri, 3 Mar 2006 07:36:14 +0100 Organization: IDI/NTNU Lines: 36 Message-ID: References: <1140915349.609443.285650@e56g2000cwe.googlegroups.com> <1140916287.800990.284570@t39g2000cwt.googlegroups.com> <1140928045.223226.301140@i40g2000cwc.googlegroups.com> <440185c0$0$11076$e4fe514c@news.xs4all.nl> <1140963712.820976.72300@u72g2000cwu.googlegroups.com> <4401cd8b$0$11062$e4fe514c@news.xs4all.nl> <1140981916.897576.34490@u72g2000cwu.googlegroups.com> <1141000908.277461.101140@t39g2000cwt.googlegroups.com> <1141180942.125742.288830@j33g2000cwa.googlegroups.com> <1141239957.788010.239620@j33g2000cwa.googlegroups.com> <4fDNf.1833$%a2.1316@trndny05> <1141311212.487197.167020@t39g2000cwt.googlegroups.com> <1141320627.995968.219210@i40g2000cwc.googlegroups.com> NNTP-Posting-Host: coleburn.idi.ntnu.no Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit X-Trace: orkan.itea.ntnu.no 1141371378 9495 129.241.111.99 (3 Mar 2006 07:36:18 GMT) X-Complaints-To: usenet@itea.ntnu.no NNTP-Posting-Date: Fri, 3 Mar 2006 07:36:18 +0000 (UTC) User-Agent: MicroPlanet-Gravity/2.60.2060 Xref: dp-news.maxwell.syr.edu comp.databases.theory:37164 In article <1141320627.995968.219210@i40g2000cwc.googlegroups.com>, jog@cs.nott.ac.uk says... > Jon Heggland wrote: > > If you instead say that a "cell" can and must contain exactly one value, > > *but* that value may very well be a list, or a set, or a relation---then > > there are no problems (in theory). > > Neither of these. I was referring to a case where specifiying an > attribute-name may map to more than one "cell" as you refer to it. i.e. > not a one-one functional mapping but a generalised one-many mapping. I don't really understand. You mean several columns/attributes with the same name? > > Join is crucial, and depends on equality between "cells". But with > > multiple values, how do you define equality? > > Well this has to be specified to the system as with all equality > matches. Is 15.0 equal to 15? Is "HELLO" equal to "Hello"? How is a > date greater or less than another? These subjective equality decisions > are already being made in numerous domains without significant issue.. > > In a sense this is no different in nature to something such as a C++ > sort template where one has to specify a comparison function. Here, > when a type is created one can define how comparisons are made, and I > don't see a problem there. As long as the type of a bunch of values is well-defined, there is indeed no problem---indeed, that makes the two alternatives I outlined equivalent. However, if any attribute is a set (or list), you probably want (for convenience) to treat it as a single value of its constituent type when it contains only one value, I.e. "Jon" = { "Jon" }. A language that does this may exhibit undesirable behaviour in other places; I believe SQL has trouble with table literals because of this. -- Jon