Re: Does entity integrity imply entity identity?

"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?

>
> 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.
>
> The entity integrity rule is generally presented as a subrule under Codd's
> rule 10. I do not know if Codd used the term "entity integrity" or not.
> And I also do not know if he explicitly made the rule that no part of a
> primary key can be missing. I started with practice rather than theory,
> and only learned enough of the theory to help me with the practice.
>
> http://en.wikipedia.org/wiki/Entity_integrity
> http://en.wikipedia.org/wiki/Integrity_constraints
> http://en.wikipedia.org/wiki/Codd%27s_rules
>
> (sorry this is reverse order).
>
>
>
Received on Tue Aug 04 2009 - 06:54:45 CEST