Path: text.usenetserver.com!out01b.usenetserver.com!news.usenetserver.com!in04.usenetserver.com!news.usenetserver.com!nx02.iad01.newshosting.com!newshosting.com!post02.iad01!post01.iad01!news.aliant.net!not-for-mail Date: Tue, 30 Oct 2007 09:21:15 -0300 From: Bob Badour User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 Newsgroups: comp.databases.theory Subject: Re: One-To-One Relationships References: In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Lines: 24 Message-ID: <472721bc$0$14846$9a566e8b@news.aliant.net> NNTP-Posting-Host: 142.176.58.53 X-Complaints-To: abuse@aliant.net Xref: usenetserver.com comp.databases.theory:167056 X-Received-Date: Tue, 30 Oct 2007 07:21:17 EST (text.usenetserver.com) Phil Reynolds wrote: > 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 that is not a one-to-one relative cardinality. That is actually a one-to-zero_or_one relative cardinality. 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. > > Thank you. I have seen no theory-based arguments for partitioning a relation into two relations with one-to-one relative cardinality. At the same time, I haven't seen any compelling theory-based arguments against doing so either. After all, with join and project one can express either from the other.