Re: Does entity integrity imply entity identity?

From: Bob Badour <>
Date: Tue, 04 Aug 2009 15:58:05 -0300
Message-ID: <4a7884c6$0$23751$>

Keith H Duggar wrote:

> On Aug 4, 10:39 am, Bob Badour <> wrote:

>>Mr. Scott wrote:
>>>"Walter Mitty" <> 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
>>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?

Rule 3 is absolutely iffy. I tend to consider Rule 4 solid. Some might question the necessity of Rule 4, and I know I have some serious reservations about the catalog design Codd proposed in RM/V2, but it seems perverse to me to deprive those who need to manage data the best tools to manage the data about the managed data. Received on Tue Aug 04 2009 - 20:58:05 CEST

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