Re: One-To-One Relationships

On 3 dec, 13:46, Cimode <cim..._at_hotmail.com> wrote:
> On Dec 3, 11:00 am, Jan Hidders <hidd..._at_gmail.com> wrote:
>
> > On 2 dec, 19:40, Cimode <cim..._at_hotmail.com> wrote:
>
> > > On 2 déc, 16:11, Jan Hidders <hidd..._at_gmail.com> wrote:
>
> > > > On 2 dec, 14:16, Cimode <cim..._at_hotmail.com> wrote:
>
> > > > > On 2 déc, 10:36, Jan Hidders <hidd..._at_gmail.com> wrote:
>
> > > > > > On 1 dec, 06:26, vldm10 <vld..._at_yahoo.com> wrote:
>
> > > > > > > On Nov 30, 9:34 am, Jan Hidders <hidd..._at_gmail.com> wrote:
>
> > > > > > > > Why by only one attribute? Why not by a set of attributes? Or a
> > > > > > > > combination of attributes and relationships (as is the case for weak
> > > > > > > > entities)?
>
> > > > > > > This is OK, but my advice to you -don't use it often.
> > > > > > > I will give you one example:
> > > > > > > The relation has A1, A2, A3, A4 "attributes" and they are mutually
> > > > > > > independent (i.e. they are in BCNF)
> > > > > > > The "attributes" can change their values for "entity" like in
> > > > > > > "temporal DB". User needs on line all information for any "entity" in
> > > > > > > any moment.
> > > > > > > Can you please write the key for this relation so that we can discuss
> > > > > > > it.
>
> > > > > > You do realize we were talking about ER modeling, not RM modeling,
> > > > > > don't you?
>
> > > > > I am talking about neither of the two. If you ask me, I place terms
> > > > > like *entity* on the same level than terms like *object*: too
> > > > > subjective to be reliable to build logical reasonning onto.
>
> > > > Any data model is to some extent subjective since it represents the
> > > > universe of discourse of a certain group of people. Of course the
> > > > representation of that data models should be exactly defined, but that
> > > > doesn't require that the notion of entity is more defined than it is
> > > > now.
>
> > > > > But I should phrased my question otherwise: what's more elementary/
> > > > > best than a name to identify an entity, any entity ?
>
> > > > Your question seems to assume that all things that we can speak about
> > > > always have a unique name. For a particular universe of discourse that
> > > > may or may not be the case.
>
> > > I am not assuming anything.
>
> > Good. Then the answer to your question is simple. If the entity
> > doesn't have a name already in the UoD then it is better to not have a
> > name to identify it in your model.
>
> I am sorry I do not understand your answer.
>
> I have *not* asked the question: *what would happen if the name is not
> an attribute of the entity?* but *what's best/more elementary than a
> name to identify an entity from another entity?*

The more elaborate answer is that the best thing is that which comes closest to how this is done in the UoD that you are trying to describe. After all, a model is better if it more closely describes the thing that it tries to describe (presuming it stays at the right abstraction level and doesn't come too close). So if things are identified by an elaborate description that involves several attributes, or even several facts about the entity, then this is what you should describe in your model. It may look more simple to invent your own naming scheme, but a more simple model is not necessarily a better model.

Does that make sense to you?

> It seems we both have a communication problem...I am truly trying hard
> to give a chance to E/R modeling...

Which is very much appreciated.

• Jan Hidders