Re: One-To-One Relationships

On Dec 2, 11:05 am, Jan Hidders <hidd..._at_gmail.com> 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.

Agreed. But then two wrongs don't make a right ;) Just because neither E/R nor RM cope too well with this sort of thing doesn't mean its beyond us to formulate something that can...

>
> > To denote the is new
> > information, well I now longer haver a binary relationship, but a
> > ternary one, and that requires a rewrite of the E/R representation
> > (given that it is a graph). I need a new lecture associative entity
> > with 'day' as an attribute.
>
> It seems you talking about a particular ER dialect here. In many, and
> certainly not in the classical one, there is no distinction between
> binary and ternary relationships.
>
> -- Jan Hidders
Received on Sun Dec 02 2007 - 13:40:14 CET