Re: Does entity integrity imply entity identity?
Date: Tue, 4 Aug 2009 11:40:22 -0700 (PDT)
On Aug 4, 10:39 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> Mr. Scott wrote:
> > "Walter Mitty" <wami..._at_verizon.net> wrote in message
> > > Also the way I learned it, entities and relationship among them are part
> > > of what was called the "conceptual data model". The conceptual data model
> > > was not a relational model as such, although it's very straight forward to
> > > start with an ER model and transform it into a relational model that
> > > expresses the same facts.
> > > The conceptual model was used for data analysis resulting in a clearer
> > > definition of the information requirements. The conceptual model was NOT
> > > a design model.
> > > The design model could be broken into two layers: logical model and
> > > physical model. I could go into more detail here, but it doesn't pertain
> > > to your question. Suffice it to say the logical model should be
> > > relational if the goal is to design a relational database. Back in 1984,
> > > they were quite loose about what was considered a relational DBMS. It was
> > > that looseness that led Codd to formulate the 12 rules, in order to
> > > distinguish between soi disant relational DBMSes and really relational
> > > DBMSes.
> I once heard speculation that the Computerworld articles where Codd
> published the 12 rules may have been more-or-less a commissioned piece
> intended to paint one vendor in particular in a positive light. While
> one or two of the rules look a little iffy in retrospect, nevertheless,
> one can gain a lot of insight from the remainder.
> In particular, one can get a lot of insight from understanding the
> logical contradictions caused by the iffy rules.
Forgive my ignrance; which are the rules are the iffy ones? I'm thinking Rule 3 and possibly 4?
KHD Received on Tue Aug 04 2009 - 20:40:22 CEST