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

On 20 jul, 18:52, kaja_love..._at_yahoo.com wrote:
> On Jul 19, 6:24 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
>
>
>
> > On 15 jul, 16:37, kaja_love..._at_yahoo.com wrote:
>
> > > 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
>
> > My apologies for the late reply. I'm currently travelling abroad and
> > am visiting another university where my internet access and time is
> > more limited than usual.
>
> > > 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?!
>
> > Yes, as it is generally is understood this correct, but note that in
> > theory it could be if there were a DBMS that used the data model that
> > is usually used for conceptually modelling as its native data model.
> > In fact, you could describe the whole effort on object-oriented
> > databases as an attempt to bring the two closer together.
>
> > > 2)
> > > There’s a little ambiguity here – can’t the term logical model
> > > represent two things:
>
> > > a)      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
>
> > Yes. Database researchers often tend to use it in that meaning. This
> > is related to the fact that these people often don't accept that a
> > conceptual model is a real data model but rather an informal and loose
> > shorthand for the real thing.
>
> > > b)      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  )
>
> > Yes, that is more how the data modelling and software engineering
> > people see it.
>
> > > 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?
>
> > As far as I can see there is no easy rule of thumb here, and even the
> > the rough distinction in different groups and meanings is only an
> > approximation of the truth, which is a bit messier. As always, if you
> > want to make sure the audience you have in mind understands you
> > correctly then you either have to make sure in advance how they
> > understand the term or be explicit yourself about how you use the term
> > (by either giving your own definition or referring to an authorative
> > one).
>
> > -- Jan Hidders
>
> >My apologies for the late reply. I'm currently travelling abroad and
> >am visiting another university where my internet access and time is
> >more limited than usual.
>
> No problem mate
>
> Can I ask just one more questions just to see if I understood it
> correctly?
>
> We often use ER-model to represent referential data models. Now if we
> create ER model for referential DB that we plan to create, then this
> ER model is conceptual model and not logical model, since it doesn’t
> deal with structures which our DB will implement?!

Yes.

• Jan Hidders