Re: Relation or attribute and why

"Marshall" <marshall.spight_at_gmail.com> wrote in message news:1147830705.849167.238130_at_u72g2000cwu.googlegroups.com...
> dawn wrote:
> > Let's say that we have a noun present in a conceptual data model, but
> > not identified as a strong entitiy (recognizing not all conceptual
> > modeling techniques use such distinctions). What are the conditions
> > under which this noun will/should translate into an attribute in a
> > logical data model? What are the conditions under which this noun
> > will/should translate into a relation in the logical data model?
>
> I searched for a definition of "strong entity" and found this page:
>
> http://www.siue.edu/~dbock/cmis450/3-ermodel.htm
>
> which said:
>
> "A Strong Entity is one that exists on its own, independent of other
> entities.
> A Weak Entity is one whose existence depends on another entity. This
> means an occurrence of one entity cannot exist unless there is an
> occurrence
> of a related entity."
>
> If I read this correctly, it's saying that a strong entity is a
> conceptual
> entity that would be modelled with a table without a foreign key.
>
> Let's say I have a Customers table; an unremarkable example.
> Probably every Customer must have at least one Address.
> Likewise, every Address belongs to a specific Customer.
> Thus, the Customer cannot exist without the Address, and
> the Address cannot exist without the Customer. Thus, both
> are weak entities. At this point, I'm about ready to conclude
> that strong vs. weak is a term that is sufficiently fuzzy as
> to be at best evocative, and at worst simply distracting.
>
> I never learned any formal modelling technique; I picked it up
> on the job. Sometimes this feels like an advantage.

Marshall,

I sorta picked up a little training in the process of learning modeling on the job. Both are valuable, IMO.

I don't like the Customer and Address example. An Address could "exist" without a customer. Let's say we have a customer who does business at 1234 Main St. Now let's say the customer moves out, and no one moves in because the building is condemned. The Address still "exists". Whether that address could or should be stored anywhere in our database is NOT the same question as whether that address "exists".

However, let's change the focus: A "Customer Address" is a weak entity. No customer, no customer address. End of discussion.

Conceptual modeling raises some issues at the existence level that turn out to be moot at the database design level. That is why so many people in this newsgroup are dismissive of conceptual modeling. They think that the only useof concpetual modeling is as a preview of logical design. That's the way I looked at ER modeling for about a year after learning it.

It wasn't until I saw an ER model of a sub enterprise that was modeled logically in a non relational DB that I really understood what a conceptual model was for. Received on Wed May 17 2006 - 06:03:48 CDT