Re: Does entity integrity imply entity identity?
Date: Wed, 5 Aug 2009 08:33:28 -0400
"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
> Mr. Scott wrote:
>> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>>>"Mr. Scott" <do_not_reply_at_noone.com> wrote in message
>>>>Since the entity integrity rule ensures that a relational table cannot
>>>>have any duplicate rows, does that imply that each row in a table maps
>>>>to a distinct entity?
>>>Here's the way I learned it, back in 1984.
>>>Each row in a table represents either an instance of an entity or an
>>>instance of a relationship between or among entities. I'm not sure
>>>whether or not "represents" in my wording is equivalent to "maps to" in
>> Isn't an instance of a relationship between or among entities also an
> > Pretty pictures... bah! > >>>>distinguish between soi disant relational DBMSes and really relational
>>>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
> > 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. > >
>>>The entity integrity rule is generally presented as a subrule under
>>>Codd's rule 10.
> > I don't know why you would say that when the description of the so-called > "entity integrity" basically restates Rule 2 verbatim.
I don't think that's true. Although the entity integrity rule implies the guaranteed access rule, the guaranteed access rule doesn't imply the entity integrity rule. The guaranteed access rule doesn't say anything at all about nulls.http://en.wikipedia.org/wiki/Integrity_constraints
Received on Wed Aug 05 2009 - 14:33:28 CEST