Re: One-To-One Relationships
Date: Mon, 3 Dec 2007 04:40:14 -0800 (PST)
On 3 dec, 12:05, Jan Hidders <hidd..._at_gmail.com> wrote:
> On 2 dec, 16:50, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> > Jan Hidders wrote:
> > > On 2 dec, 16:10, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> > >>Jan Hidders wrote:
> > >>>On 1 dec, 15:03, JOG <j..._at_cs.nott.ac.uk> wrote:
> > >>>>[...] let me give an example as to why I find
> > >>>>the breakdown into entities and relationships deleterious. Say I have
> > >>>>two entity types staff_members and subjects, and a relationship
> > >>>>teaches:
> > >>>>staff_member -- teaches --> subject
> > >>>>This is all good and fine until a requirement changes that we need to
> > >>>>record the day the lecture is given on.
> > >>>Yes. Relationships may become Entities, Roles may become
> > >>>Relationships, Attributes may becomes Relationships, etc. As David
> > >>>remarked, there are ER dialects like ORM that smoothen this a bit, but
> > >>>you can never make that really go away. These problems have
> > >>>counterparts in the RM where foreign keys may become tables by
> > >>>themselves, or tables are split because a one-to-one relationship has
> > >>>become a one-to-many one.
> > >>ORM = ER ?!? Since when?
> > > Since the objects and the fact-types in ORM are basically the same
> > > thing as entities and relationships in ER.
> > Basically the same? Or the same?
> That depends on which ER dialect you are talking about.
To say a bit more about this, the notion of "being the same" is not a straightforward one here. If you just look at the formal definitions then these are exactly the same, but that is not the only thing that determines if the notions are really the same because also their usage (i.e. how you decide if and how to model something with these constructs) may play a role in whether you consider them as really the same or not. However, that is inherently always informal and so if that is what you meant then a definitive answer to your question is neither interesting nor really possible.
- Jan Hidders