Re: Does entity integrity imply entity identity?
Date: Wed, 5 Aug 2009 08:33:28 -0400
Message-ID: <c5ydne_wGO2E4eTXnZ2dnUVZ_jydnZ2d_at_giganews.com>
"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
news:4a784836$0$23766$9a566e8b_at_news.aliant.net...
> Mr. Scott wrote:
>> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>> news:JTadm.209$nh2.42_at_nwrddc02.gnilink.net...
>>
>>>"Mr. Scott" <do_not_reply_at_noone.com> wrote in message
>>>news:IaKdnaZV7ZxZyOnXnZ2dnUVZ_uednZ2d_at_giganews.com...
>>>
>>>>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
>>>your wording.
>>
>> Isn't an instance of a relationship between or among entities also an
>> entity?
> > 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
>>>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. > >
>>>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.
>>>>http://en.wikipedia.org/wiki/Integrity_constraints
>>>http://en.wikipedia.org/wiki/Entity_integrity
>>>http://en.wikipedia.org/wiki/Codd%27s_rules
Received on Wed Aug 05 2009 - 14:33:28 CEST