RE: [topicmaps-comment] TAO vs. ERA

[Scott Tsao <skst4xml@yahoo.com>]

--- Marc de Graauw <marc@marcdegraauw.com> wrote:
> [...] So an occurrence is certainly not the same
thing as an
> attribute. An attribute is information that is
stored in the relational
> database, whereas an occurrence is 
> a piece of information which may or may not be in
the Topic Map.

Considering that XLink uses attributes exclusively for
resource linking, I tend to think that there is a
direct mapping between TAO's occurrence and ERA's
attribute.

> [...]
> Entity-Relationship-Attribute (ERA) is a
> datamodelling technique, I think Chen 
> developed it first but I'm not sure.

I think so.  Peter Chen is known as the originator of
ER Model (see http://www.csc.lsu.edu/~chen/chen.html).
 He is also a member of the XML Schema WG which
adopted the ERA techniques for mapping between
relational models and XML documents.

> In ERA you have entities (which are called 
> relations in the relational model) which contain
> attributes and relationships, 
> which are 'associations' between two or more
> entities (and which may or may not 
> contain attributes, depending on the flavor ERA you
> are using). So in ERA there 
> are things like associations (TM terminology) or
> relationships between 
> entities, and in ERA it is possible have
> many-to-many relationships directly, 
> as well as ternary (and other n-ary) relationships.

Thanks for pointing this out.  I was trying to compare
ERA, not RDB (relational database), and TAO.

> In general I think the comparison topic=entity,
> association=relationship and 
> occurrence=attribute is misleading. I think
> occurrences are definitely _not_ 
> the same thing as occurrences, because occurrences
> may be outside the Topic Map 
> and may even be offline resources.

I think you meant to say "I think occurrences are
definitely _not_ the same thing as 'attribute'." 
However, I maintain that there is a good reason to see
occurrences (links) as attributes.  (see above)

> As for the topic-entity identity, I think 
> that may or may not be the case. When a topic is
> 'person' this could very well 
> be an entity in an ERA model (table in a database),
> but when a topic is 'Marc 
> de Graauw', this would normally be a row in a
> relational database, not a table. 

I think what you are pointing out here is the
difference between "class" and "instance".  The
comparison between TAO and ERA should be done within
the same context, i.e., Topic Type vs. Class of
Entity, or, Topic vs. Instance of Entity, etc. 

> [...]
> As for the differences and strengths of the
> relational model and Topic Maps I 
> would think that the relational model is the better
> approach when one wants to 
> store structured data in a computer. That is what
> the relational model and 
> relational databases were developed for, and what
> they're good at. [...]

I suspect that this discussion may lead to the
unveiling of internal mapping of RDB-based TopicMaps
engines, which could be well-guarded secret of
vendors.

Thanks for the thoughtful comments.

Scott

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