Re: Relation or attribute and why

Gene Wirchenko wrote:
> On 23 May 2006 15:44:15 -0700, "dawn" <dawnwolthuis_at_gmail.com> wrote:
>
> >Gene Wirchenko wrote:
> >> On 23 May 2006 13:44:29 -0700, "dawn" <dawnwolthuis_at_gmail.com> wrote:
> >>
> >> [snip]
> >>
> >> >In my example, the conceptual data model includes: name, firstName,
> >> >lastName with relationships such that name has-a firstName and name
> >> >has-a lastName.
> >>
> >> That is part of the logical model. The conceptual model is the
> >> business statement.
> >
> >The identification of name, lastName, firstName and the relationships
> >among these terms is not part of the business scope and definition, the
> >conceptual data model? What would the CDM include related to these
> >terms? I put everything that is conceptual and not directed to a
> >particular implementation model (such as the RM) in the CDM. Where do
> >you draw the (possibly fuzzy) line? --dawn
>
> Possibly fuzzy, but nonetheless.
>
> Conceptual might fit on a napkin: "We want an E-commerce Web site
> for selling our products. It has to be able to handle North America,
> but we plan to go international, so have the capability to add other
> languages, etc. easily without rewriting large chunks. We want
> someone able to complete an order quickly. Do not forget good
> security. OtherCorp recently had a bad situation, and they are taking
> a kicking. Now is our chance, if we do it right."

Those are high level requirements, but not a conceptual data model. Conceptual, yes; CDM, no. You need at least an ORM, ERD, or a cleaner list of propositions related to these requirements, I would think.

>
> Logical gets into the details, but not the implementation.

A typical use of the term would allow for a conceptual data model to go from high level to detailed (in the end). [Although if a decision is made earlier that the requirements have been captured sufficiently to do flesh it out using common sense in a transition to an LDM, I can live with some agility.]

I once considered the LDM to be implementation-independent, but after reading other definitions the logical data model appears to be data model dependent to most who use the term. In that case, you would not have the same logical data model for an RM implementation as an MV implementation, for example.

>From your definition, there might be an implementation data model
between the logical data model and the physical data model, if I am understanding you correctly (unless you are calling the specification to the DBMS "the physical data model"). The LDM would then be data model independent (so it might have lists, even if the impl was the RM, for example), while the Implementation data model would align not only with a data model implementation but also a dbms tool. The physical data model would be employed by the DBMS, of course, and is not something I consider, for the most part.

Whatever the terms, I'm interested in going from a final data-model-independent conceptual data model to a model from which CREATE TABLE statements can be handily derived (for an RM implementation). Then I can better understand and communicate the differences in modeling for another data model. Thanks. --dawn Received on Wed May 24 2006 - 03:08:29 CEST