Basic ANSI SQL inherent hierarchical processing using the Left Outer Join to model and processes hierarchical structures is basically quite obvious and empirical and I have covered this in my previous SQL/XML articles. The proof for multipath hierarchical query processing which requires Lowest Common Ancestor (LCA) processing occurring naturally in ANSI SQL is not that obvious because it is truly quite amazing since it was never designed into ANSI SQL and is quite complex to perform. Empirically it can be proven from its results, but it would be very nice to know how and why it is working so we can absolutely trust the results. I have written an article describing the how and why of natural LCA processing in ANSI SQL and have appropriately entitled it “The Ghost in the Machine”. It can be located below. 

The Ghost in the Machine
http://www.tdan.com/view-articles/11069

This LCA processing in XQuery is not automatically performed today and is too complex to do with procedural navigation. This problem has been researched academically and attempted solutions use LCA functions that have to be inserted correctly by the query user which takes away for its ease of use and schema-free purpose. My work with LCA processing has shown that LCA processing can involve nesting LCA’s that I do not necessarily see occurring in this LCA XQuery research limiting their future solutions to more simple queries. ANSI SQL performing LCA processing automatically has no multi-path LCA query limitations. This has been referred to a LCA query processing, at least three decades ago.

 Regards,
 /Mike

Michael M David
Advanced Data Access Technologies, Inc.
www.adatinc.com