Dear Matthew, (01)
I agree that the distinction is essential, but that approach
makes it difficult to express all necessary constraints: (02)
> MW: Actually it was a modelling principle, that entity types should be
> based on the underlying nature of a thing rather than a role that it
> plays.
>
> MW: Following this approach relations were also modelled as entity types
> and roles were modelled on the relationships (E-R sense) linking the
> relations to the domains of the things they related. (03)
But I put all types in a single hierarchy, in which all roles are
under the type Role. Since the hierarchy is a lattice, it allows
all possible combinations, such as MaleEmployee or PregnantEmployee. (04)
This approach makes it possible to state constraints on
permissible relationships very conveniently. For example,
a relationship that is restricted to PregnantEngineerEmployee
can be hard to represent in a typical E-R diagram. (05)
John (06)
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