Re: Has E/R had a negative impact on db?

"Jon Heggland" <jon.heggland_at_idi.ntnu.no> wrote in message news:e2hslc$sfq$1_at_orkan.itea.ntnu.no...
> -CELKO- wrote:
> > One problem is that a relationship in a ER diagram is a line between
> > boxes. That puts us in a binary-only world. ORM, on the other hand,
> > allows n-ary relationships.
>
> Are you talking about Chen's original ER? Because most ER variants I've
> seen definitely allow n-ary relationships---and the relationship aren't
> the lines, they're the diamonds.

Agreed.

And another way to depict n-ary relationships in an ER model is called "reification". That's just a fancy word for expressing a relationship as if it were an entity. The reified relationships play the role of diamonds in the above.

The UTexas outline of modeling suggests doing this for resolving many-to-many binary relationships. They call it "association entities". I don't doo what they suggest, but I have done it for n-ary relationships.

A second parallel question is whether attributes can be attached to both entities and relationships, or whether they can only be attached to entites. I've followed the first practice.

In my own mind, without developing any formal concept for it, I've had a tendency to view entities as "unary relationships". I realize that's an oxymoron, but it's intentional. This collapses two concepts, entity and relationship, down to a single concept, relationship. Received on Mon Apr 24 2006 - 14:33:28 CEST