kaja_love160_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
>
> 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:
>
> 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
>
> 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 )
>
> 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?
PMFJI, network and hierarchic dbmses do not use tables, but they still
have logical models. Not knowing as much about the ANSI/SPARC
terminology, I am interested to hear what Jan has to say.
Received on Sun Jul 15 2007 - 10:32:37 CDT
