Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!postnews.google.com!i39g2000cwa.googlegroups.com!not-for-mail From: "David Portas" Newsgroups: comp.databases.theory Subject: Re: circular relationships ok? Date: 3 Mar 2006 13:00:26 -0800 Organization: http://groups.google.com Lines: 19 Message-ID: <1141419626.854405.149500@i39g2000cwa.googlegroups.com> References: <440705d2$1@news.fhg.de> <1141312379.065049.193100@p10g2000cwp.googlegroups.com> <440718b9$1@news.fhg.de> <1141317138.737621.294520@i40g2000cwc.googlegroups.com> <1141318257.897955.150140@v46g2000cwv.googlegroups.com> <440727b9$1@news.fhg.de> <1141333040.551974.105530@e56g2000cwe.googlegroups.com> <440802f8$1@news.fhg.de> NNTP-Posting-Host: 86.137.91.147 Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" X-Trace: posting.google.com 1141419632 11560 127.0.0.1 (3 Mar 2006 21:00:32 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Fri, 3 Mar 2006 21:00:32 +0000 (UTC) In-Reply-To: <440802f8$1@news.fhg.de> User-Agent: G2/0.2 X-HTTP-UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 2.0.50727; InfoPath.1),gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: i39g2000cwa.googlegroups.com; posting-host=86.137.91.147; posting-account=GH1roQ0AAACgftr1B9S4n3XwqdsCU7E1 Xref: dp-news.maxwell.syr.edu comp.databases.theory:37212 Alexandr Savinov wrote: > Now for each order there as an invoice and vice verse (we need of course > a constraint to enforce one-to-one relationship). So you DO allow constraints that are cyclical. This constraint (called "N-cardinality", where N=1) applies "in both directions". In a conventional ER diagram this would be shown as a cycle. Your diagrams on the other hand don't seem to model constraints - they show something else that isn't clear to me. That something else doesn't have any cycles. The OPs original question was specifically about cycles in referential integrity constraints, which you do say you need in your model (you just don't draw pictures of them). -- David Portas