Path: text.usenetserver.com!out04a.usenetserver.com!news.usenetserver.com!in02.usenetserver.com!news.usenetserver.com!cycny01.gnilink.net!spamkiller2.gnilink.net!gnilink.net!trndny07.POSTED!3abab865!not-for-mail From: "David Cressey" Newsgroups: comp.databases.theory References: <0aIVi.160673$1y4.23786@pd7urf2no> Subject: Re: One-To-One Relationships Lines: 53 X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 6.00.2800.1478 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1478 Message-ID: Date: Tue, 30 Oct 2007 19:57:39 GMT NNTP-Posting-Host: 72.79.204.207 X-Complaints-To: abuse@verizon.net X-Trace: trndny07 1193774259 72.79.204.207 (Tue, 30 Oct 2007 15:57:39 EDT) NNTP-Posting-Date: Tue, 30 Oct 2007 15:57:39 EDT Xref: usenetserver.com comp.databases.theory:167072 X-Received-Date: Tue, 30 Oct 2007 14:57:40 EST (text.usenetserver.com) "paul c" wrote in message news:0aIVi.160673$1y4.23786@pd7urf2no... > David Cressey wrote: > > "Phil Reynolds" wrote in message > > news:CGzVi.2012$%13.1165@newssvr22.news.prodigy.net... > >> One thing that's not clear to me is when it's appropriate to create a > >> one-to-one relationship. I mean, in some cases it's obvious, if there's a > >> set of data that wouldn't always apply; then you'd want to create that set > >> of fields in a separate table with a one-to-one relationship. But in what > >> other cases? After the number of fields in a table is greater than X? > >> > >> I'm just curious about what thoughts/theories/ideas people have about > >> one-to-one relationships, because that's something that's never been > >> entirely clear to me. > >> > > > > I think that a relationship is something you discover, not something you > > create. Are you talking about creating two tables where there is a > > one-to-one relationship between rows in table A and rows in table B? If > > so, I think what you have created is not the relationship as such, but a > > way of representing it in the database. > > > > I hope this isn't too nit picky. I think the distinction between what you > > discover via analysis and what you create during implementation (following > > design) is very fundamental, and needs to be kept clear in all our > > discussions. > > > > > > > > I don't think it's too nit picky at all. I wish the OP had given an > example because I think people here are talking about two different > things as you suggest, ie., a one-to-one as the ER people would see it > (eg., dept has one mgr and mgr has one dept) versus a relation that > somebody wants to make into two relations. > Thanks. It gets even foggier because many of us (myself included) use the term "relation" in reference to something that, in the strictest mathematical definition, is really a "relationship".