Re: Relation or attribute and why

Marshall wrote:
> 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.

Wikipedia, for its sins, deems a weak entity to be an entity that cannot be uniquely identified by its own attributes alone and must hence use a foreign key in conjunction with its attributes to create a primary key.

Leibniz must be turning in his grave.

>
> I never learned any formal modelling technique; I picked it up
> on the job. Sometimes this feels like an advantage.

After a formal E/R education, ORM is my new mistress, so I can heartily say I don't think you've missed out on much.

>
> Anyway, to take a stab at your actual question, I would
> say the thing to attend to, bearing in mind that I'm speaking
> quite informally, is the cardinality of the "noun."* If it's
> exactly-one
> with a "strong entity" then it's an attribute of that strong entity.
> Otherwise it's part of some other "sub" entity.
>
>
> Marshall
>
> * A noun is a kind of word, so I'm pretty sure you meant
> something else. </picky>

I believe Dawn is trying to avoid the term 'thing' to describe an entity - I don't blame her for this. The fact that the term is a vital syllable in the phrase "absolutely bloody any-thing at all" highlights its lack of utility imo. Received on Wed May 17 2006 - 04:30:59 CEST