Re: What is that "more" that makes E-R model truly independent ?
Date: Tue, 24 Jul 2007 08:44:34 -0700
Message-ID: <1185291874.325737.75510_at_w3g2000hsg.googlegroups.com>
1)
On Jul 24, 1:22 pm, "David Cressey" <cresse..._at_verizon.net> wrote:
> "beginner16" <kaja_love..._at_yahoo.com> wrote in message
>
> news:1185239310.147908.316170_at_n60g2000hse.googlegroups.com...
>
> > 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.
>