Re: Clean Object Class Design -- What is it?

> >In a logical model an OID is a value (who says it's a pointer?), in the
> >physical model it could be a pointer or something completely different.
 In
> >what fundamental manner does an 'object identifier' differ from a
 'primary
> >key' in the relational model?
>
>
> Well, for a start, one cannot use the same OID 'value' to identify
 multiple
> things,

The relational model requires that a PK k in a Relation R i.e. the combination (R, k) uniquely identifes a single tuple. We could apply an analagous minimal requirement in a conceptual object model that an OID uniquely identifies an instance within a class extent. System wide or (uinversal) uniqueness is a stronger but not necessarily essential constraint.

> and most systems do not treat them symmetrically.

Can you please explain what you mean by this? Received on Mon Jul 23 2001 - 11:40:03 CEST