Re: erd to db

"Michael Gruebsch" <michael.gruebsch_at_pentaprisma.com> wrote in message news:a7nhlg$mn178$1_at_ID-71391.news.dfncis.de...
> Thanks Jan, for answering. I'm afraid I was not clear. I meant the
following
> constallation
>
> [BOOKING] -- (N) -- <ROLE> -- (M) -- [ADDRESS]
>
> where the relationship <ROLE> have an attribute (TYPE). Both entities
> BOOKING and ADRESS are strong entities having both a primary key
> consisting of an attribute (IDENT) for the booking number or customer
> id resp.
>
> Transforming this into relational schema <ROLE> can be modelled by
> an entity [ROLE] having a (primary) key consisting of the attributes
> (BOOKING.IDENT, ADDRESS.IDENT) at least.

Brrr, I feel these shivers running down my spine again. :-) So if you allow me to be pedantic for a moment: in the relational model there are no entities, there are relations.

> The question is: How can I model the ERM that the transformation into
> the RS includes (TYPE) into the primary key of [ROLE]? There should
> be a notion of a key for a relationship ... iff "multi-valued"
relationships
> are allowed.

In this case you model it by introducing an entity type TYPE and model ROLE as a ternary relationship between BOOKING, ADDRESS and TYPE. However, in general it is in many ERD techniques not possible to add other keys than just the collection of all the involved roles. A notable exception is ORM which is IMHO the best technique anyway.

• Jan Hidders