Re: [topicmaps-comment] TAO vs. ERA

["Thomas B. Passin" <tpassin@home.com>]

[Chris Angus]

> When you define a column in a relational table you are inherently defining
> two classes (types).  The first defines the domain of the column from
which
> values may be drawn - one can think of it as the type of the column.  The
> second is a subtype of the first and is the set of things that play the
role
> that is implicit in the column in one or more tuples in the relation.

Yes, this is the classic range-domain thing, which is not explicitly
supported in topic maps.  Of course,there is nothing to stop you from
declaring a topic with the range and domain as occurrences (for example - it
could be done with associations) and using that to define the range and
domain.  This would be an example of one of those "idioms" I posted about a
few weeks ago.  It would be hard for a general-purpose processor to
understand the "meaning" of such a construction, but otherwise the fact that
you could do it is a reflection (in my mind at least) of the flexibility of
topic maps.

> The relational model itself does not provide a specific mechanism for
saying
> much more about the role, beyond using a suitable name for the column, but
> nevertheless the class is there.
>
> One of the characteristics of such 'role-based' classes is that in the
> general case a member of the supertype class can be a member of any number
> of the 'role-based' subtype classes.  They therefore do not make good
> candidiate entity types (entity sets in Chen's E-R model) in an E-R-A
model
> as they are not sortal types.
>
Yes, this is very interesting and fits right in with what Bernard just
wrote.  It's obvious that it is one of the things that makes automatic
conversion into topicmaps so hard.  What is not so obvious, though, is that
good quality conceptual modeling is VERY HARD, it takes A LOT of experience,
and many systems just don't have a good underlying model.

This applies equally to E-R modeling, RDF, Topic Maps, KIF, UML, and any
other such modeling system.  How many bad E-R models have you seen?  I've
seen many!  How many times have you seen dataflow diagrams used totally
inappropriately (like when they try to be software architecture drawings)?

(BTW, I don't really advocate pushing an analogy between topic maps and
relational models too far.  I've just been musing on how you could make such
an analogy if you really wanted to)

>
> I think that one has to be careful about drawing too much of an analogy
> between E-R and topics and associations, partly because of restrictions in
> the E-R model.

D'accord.

>Among the things that I would cite are the following:
>
> (1) A relationship cannot be treated as an entity whereas an association
can
> be reified as a topic.
>
> (2) A topic can equate to either an entity set (using Chen's terminology)
or
> an entity (i.e. an entity type or an instance of an entity type) -
something
> that the E-R model does not support.
>
> (3) An association can exist between a topic that equates to an entity set
> and a topic that equates to an instance of an entity set.
>

Very well thought-out stuff here.

Cheers,

Tom P