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   Re: [xml-dev] Tree v. Table - A relational XML object model...?

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At 11:12 AM 2/27/2003 +1100, Matthew.Bennett@facs.gov.au wrote:
>I recognized in CS 101 that a binary tree is just a table, skewed 45
>degrees, a non-binary tree a sparse table, skewed 45 degrees. They are
>abstractions of the *same* concept! Hence, it ought to be just as valid to
>speak of XML tables, as XML trees.
>Perhaps  a matrix-algebra mathematician can get around to having an
>epiphany, and come up with the ..... "relational XML object model".
>Possible, or pie-in-the-sky?

Nested Relational Algebra allows hierarchy, and comes close to XML in many 
ways, but XML is ordered, which is quite significant. There's a significant 
literature on Nested Relational Algebra.

There are various Algebras for XML that have been proposed for query 
processing, and there is also a Formal Semantics used to formally define 
XQuery, an XML query language.

But anything that really supports XML well winds up being significantly 
different from relational algebra in some ways. After all, unordered two 
dimensional tables in which the rows have no inherent identity are rather 
different from ordered, labelled forests with implicit node identity and 
the ability to establish references among the nodes. XML expresses most 
things via hierarchy and sequence, which are both absent from the original 
relational model.

Of course, mapping relations to XML is much more straightforward, and is 
even covered by standards.



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