Re: Does entity integrity imply entity identity?
Date: Tue, 04 Aug 2009 11:39:42 -0300
Message-ID: <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!
>>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.
>>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/Entity_integrity
>>http://en.wikipedia.org/wiki/Integrity_constraints
>>http://en.wikipedia.org/wiki/Codd%27s_rules
Received on Tue Aug 04 2009 - 16:39:42 CEST