Re: At what ANSI/SPARC level are you, when creating new... totally lost

From: <kaja_love160_at_yahoo.com>
Date: Sun, 15 Jul 2007 07:37:28 -0700
Message-ID: <1184510248.479712.153270_at_w3g2000hsg.googlegroups.com>


On Jul 15, 10:45 am, Jan Hidders <hidd..._at_gmail.com> wrote:
> On 14 jul, 17:20, kaja_love..._at_yahoo.com wrote:
>
> > hello
>
> > This is really confusing
>
> > >>Thus, by creating a new database are you creating logical model or
> > >>conceptual model?
>
> > >The external model and the conceptual model are both logical models.
>
> > But the quote below suggest that logical model is part of internal
> > schema and thus conceptual model can't also be logical model?!
>
> > >>An internal schema is an organization of data according to the technology [...snip...]
>
> No wonder you are confused. In his article David Hay mixes ANSI/SPARC
> terminology with general data modelling terminology and pretends they
> are the same. This is not the case, especially terms such as "logical
> model", "internal model" and "conceptual model" have sometimes subtly
> different meanings. As a consequence he actually misrepresents the
> exact meaning of the ANSI/SPARC terminology. The ANSI/SPARC layers
> were meant to describe the internal archtitecture of a certain DBMS,
> and therefore, by David Hay's definition of the term, are actually
> *all* in the internal layer. What he calls the conceptual schema and
> external schema are completely outside the range of what the ANSI/
> SPARC architecture attempts to describe.
>
> > Since I only know ( a little ) about relational database, I'm going to
> > ask the following question in the context of relational database:
>
> > The way I understand the above paragraph is that logical schema
> > ( which the article claims is a part of internal schema ) deals with
> > tables, while conceptual level deals with objects ( entities ) and
> > thus knows nothing about tables and keys. Uh, what am I missing here?!
>
> As the terms are usually used in a data modelling context this is
> largely correct. Btw., entities, of course, also have keys.
>
> -- Jan Hidders

hello

I can’t thank you enough for helping me out. If I may ask you just one last thing, just so I can see if I got it right? And I apologize if my questions seem repetitive

1)
Since in the context of general data modeling, conceptual model deals only with objects ( entities ) and knows nothing about table structures etc, then that must also mean that in the context of general data modeling, conceptual model is not a logical model?!

2)
There’s a little ambiguity here – can’t the term logical model represent two things:

  1. in the broader sense of the word, logical model could be any model that abstracts the real world , even if the model is only on the paper and not intended for actual DB implementation ( for example  conceptual model in the context of general data modeling ) and thus doesn’t use table structures etc
  2. while on the other hand we are using the term logical model for actual model being implemented into DB ( thus conceptual model in the context of general data modeling would not be considered a logical model )

When do we use term as described in a) and when as described in b), or does, when talking about databases, logical model always refer to model I described in point b)  logical model being models that use table structures?

cheers Received on Sun Jul 15 2007 - 16:37:28 CEST

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