Re: Conceptual, Logical, and Physical views of data

On 8/30/2005 5:07 AM, David Cressey wrote:
> My introduction to databases, back in the 1980s, was, in part, by way of
> Martin. I got this description of multiple views of data: the conceptual,
> logical, and physical views.
>
> The conceptual view of data: the data as seen by the subject matter expert.
> The meaning of each of the values, in context, is part of the subject matter
> expert's domain. Database structure is not the domain of the SME, in that
> role.
>
> The logical view of data: the data as seen by the programmers. The people
> who store and retrieve data from a database (or who write software that does
> the same thing) view the data in terms of its appearance at the interface.
> Structural aspects of the data that are visible at the programming
> interface are part of the logical view. Structural aspects that are
> transparent at the interface are not part of the logical view. Structural
> alterations at the logical level may require program maintenance, but
> structural changes at the physical level will not.
>
>
>
> The physical view of data: the data as seen internally in the DBMS. This
> view of the data might be of concern to the DBA, the Data Base Designer, or
> to the DBMS engineer.
>
> Missing from the above is the data as seen by the user. Users are generally
> represented by the SME, though not always.
>
> My understanding of the conceptual data model, the logical data model, and
> the physical data model are influenced by the above.
>
> In particular, I don't buy the concept that the logical model is unrelated
> to inplementation. In the design process, the logical model is going to,
> for all practical purposes, determine the portion of the physical model
> that will be visible at the programming interface once construction is
> complete.
>
> Once construction is complete, the logical model is just as "real" as the
> physical model. The terminology changes with the context, but the design
> carries over. Thus "relations" will be implemented as "tables", but they
> are equally "logical".
>
> If you use SQL to construct the database, the relations in the logical
> model will appear as tables and indexes. Tables and indexes have logical
> features and physical features as well. In general, the logical features of
> tables and indexes appear inside the parentheses in the CREATE TABLE or
> CREATE INDEX statement, while the physical features appear somewhere else.
>
> The fact that indexes are visible at all in the logical view is simply a
> reflection of the fact that, in the SQL world, the programmer needs to know
> something about indexes for performance reasons.
>
> Having said that, one goal of logical data modelling is that the model be
> protable between two different DBMS products without changes to the model.
> Thus a good logical model for a relational database should be invariant on
> whether the implementation will be in DB2 or Oracle.In practice, this is not
> always possible, because choices of datatypes will be influenced by the
> choice of a target DBMS, and those choices will become part of the logical
> model.
>
>
>
>
>

I thought the 3 layers were:

1 - The conceptual which deals with end users and client programing.

2 - The logical that deals with the design on the database and maintaining the data.

3 - Th physical which is how the DBMS stores the data on disk and in memory.

Dan Received on Tue Aug 30 2005 - 21:45:12 CEST