Re: What is that "more" that makes E-R model truly independent ?

From: beginner16 <>
Date: Tue, 24 Jul 2007 08:44:34 -0700
Message-ID: <>

On Jul 24, 1:22 pm, "David Cressey" <> wrote:
> "beginner16" <> wrote in message
> > hello
> > 1)
> > Before I ask the question I must point out that I understand the
> > difference between logical and conceptual level. Thus, conceptual
> > model represents DB design independently of the underlying logical and
> > physical structure. I also realise that hierarchical and relational
> > models are at logical level!
> > I just started learning a little about E-R model. I realize the E-R
> > model was created so that we can represent DB at the conceptual level,
> > and as such DB is presented independently of the underlying logical
> > DB design.
> > Say we are trying to create DB for particular organization. At
> > conceptual level E-R model for this DB would be the same no matter if
> > logical implementation of this DB will be hierarchical or relational
> > ( I realize that this is probably not always true )
> > We could implement same graphical symbols ( let us called this set of
> > symbols with "S" ) to represent objects, relationships etc in both
> > relational and hierarchical models, but when trying to represent DB at
> > conceptual level using these symbols, ( I assume ) the two diagrams
> > ( hierarchical and relational ) would still be very different, even if
> > both relational and hierarchical models would use same graphic
> > symbols?!
> > But as I stated before, E-R model diagram would be the same no matter
> > if logical DB implementation is hierarchical or relational. For that
> > reason I assume that ER modeling is more than just using symbols for
> > entity, attributes, relationships ( since relational and hierarchical
> > models use same "fictional" graphical symbols and still their diagram
> > representations of DB at conceptual level differs ). But what is that
> > "more" that makes E-R model truly independent of logical DB
> > implementation ?
> > 2)
> > Does relational model also have its own graphical symbols defined
> > ( for graphical representation ) or do we always need "outside"
> > diagrams ( E-R model, Bachman's model etc ) in order represent it
> > graphically?
> > thank you
> > cheers
> My insight into what makes E-R valuable comes, in part, from some
> experiences dating back to the 1990s. In one case, I was working with the
> local data modeller, and he had taken a huge enterprise wide database built
> for VAX DBMS and abstracted it out to an E-R model (including, but not
> limited to, a diagram). when the time came to design and build a reporting
> database in Oracle that used some data from an Rdb database, but which
> described the same underlying enterprise as the old database, the data
> analysis phase of the project was essentially completed in an afternoon, by
> taking the suubset of the E-R model relevant to the subject at hand.
> For your background, VAX DBMS was (and still is) a CODASYL database
> product. CODASYL (or network) databases are like hierarchical except that
> a record can belong to more than one set.
> Anyway, the advantage that E-R offers over some alternatives is that it is
> less biased toward one data model or another. It's easy to derive a
> relational model from an E-R model. It's also fairly easy to derive a
> hierarchical model, a network model, or even an object oriented model from
> the same starting place. This lack of bias is, from my experience, very
> important. It helps in the necessary discipline of keeping design distinct
> from analysis.
> As to whether relational has its own diagramming convention, the answer is
> decidedly yes. There are even some tools out there that will manage an E-R
> model and an equivalent relational model in parallel. One such tool is Data
> Architect from Sybase.

Do we sometime use relational diagrams even at conceptual level( data modelling and not ANSI/SPARC ) or only at logical level?

Quote from my book:
"With conceptual model we can't ( ussually ) define semantics of data".

I know what the term semantics means, but not in this context. Does it mean that at conceptual level the diagrams don't tell us the meaning of that data? Uh, that doesn't make much sense Received on Tue Jul 24 2007 - 17:44:34 CEST

Original text of this message