Embedding Content as Element Content or As An Attribute Value

[David Megginson]

Michael Kay writes:

 > I think Marcus is wrong. The domain of application of SGML is
 > different from the domain of application of XML, and the
 > distinction between attributes and content which made sense in the
 > SGML world is extremely perplexing to those with a background in
 > data modelling and data structure design in other domains, who are
 > legitimate members of the XML community.

You are absolutely right.  The problem is that XML uses a
fundamentally different approach to data modelling than that used in
the relational world, and people accustomed to mapping their data onto
two-dimensional tables may have trouble getting used to the idea that
XML data structures (like those in object-oriented databases) can be
hierarchical, repeatable, and even recursive or circular.

 > In the DTD I've been designing, for what it's worth, I'm currently
 > using content for nearly everything, with very little use of
 > attributes. The main reason is for future extensibility; elements
 > can always acquire a richer internal structure, while attributes
 > can't. The drawbacks (e.g. inability to specify any constraints on
 > values, default values, etc) don't actually lose me much, because
 > the constraints available for attributes are very limited anyway.

This is a good approach.  Even in SQL, the built-in constraints have
little use: nearly everyone needs to write middleware to enforce
complex business rules that cannot be captured by the standard types
and constraints; likewise, XML implementations need middleware to
enforce business rules for their content, while the XML parser may
validate the structure.


All the best,


David

-- 
David Megginson                 ak117 at freenet.carleton.ca
Microstar Software Ltd.         dmeggins at microstar.com
      http://home.sprynet.com/sprynet/dmeggins/

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