Re: Relation or attribute and why

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.

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>