Re: "Structured" Entity-Relationship Model?
From: James K. Lowden <jklowden_at_speakeasy.net>
Date: Sat, 20 Apr 2013 20:28:28 -0400
Message-Id: <20130420202828.fe45b4bc.jklowden_at_speakeasy.net>
Date: Sat, 20 Apr 2013 20:28:28 -0400
Message-Id: <20130420202828.fe45b4bc.jklowden_at_speakeasy.net>
On Sat, 20 Apr 2013 22:02:46 +0200
Wolfgang Keller <feliphil_at_gmx.net> wrote:
> the essential aspect seems to be ... that the dependency graph of all
> entities must not contain directed circles
That might be usually true of BCNF designs, but it's not a rule per se. Normalization forms are defined in terms of redundancy and projection analysis, not graph theory.
Using the SERM example on the wikipedia page, the graph says every order has N order items, and every order item has 1 order. Although drawn as just one line, that line states two rules, mutually dependent: a cycle. (Try using DRI to enforce them.)
--jkl Received on Sun Apr 21 2013 - 02:28:28 CEST