XML Query Algebra - Does it Add Up?

On December 4, 2000 the XML Query Working Group published its latest specification, XML Query Algebra, which will be the basis of the query language. It borrows heavily from XPath and SQL. Earlier documents include the XML Query Requirements and XML Query Data Model, both from mid 2000. All three documents are still Working Drafts, which means you can expect numerous changes downstream. The XML Query Data Model and the XML Query Algebra together will define the exact semantics of what will be called the XML Query Language, which is yet to be published (naturally).

Take a look at the first example (of many) from the Algebra Working Draft. Although it may be elegant to a mathematician, it is something of a beast to web developers. The excerpt below is from the Iteration section; the query iterates over all books and extracts the author and title. The first two lines denote the actual query and the remainder indicates the format of the result. Clearly, much of this needs to be worked into future tools to make it useful, but I imagine that is the intent of the Working Group.

        for b in bib0/book do
                   book [ b/author, b/title ]
             ==> book [
                   author [ "Abiteboul" ], 
                   author [ "Buneman" ], 
                   author [ "Suciu" ], 
                   title  [ "Data on the Web" ]
                 ], 
                 book [
                   author [ "Fernandez" ], 
                   author [ "Suciu" ], 
                   title  [ "XML Query" ]
                 ]
             :   book [
                   author[ String ]{1, *}, 
                   title[ String ]
                 ]{0, *} 
    

In the XML Query Algebra Working Draft, the authors give homage to their influences:

"This work builds on long standing traditions in the database community. In particular, we have been inspired by systems such as SQL, OQL, and nested relational algebra (NRA). We have also been inspired by systems such as Quilt, UnQL, XDuce, XML-QL, XPath, XQL, and YaTL."

As some of you may recall, the original query language work dates back to December 1998 from QL'98 - The Query Languages Workshop which drew 66 different proposals. Interested readers may wish to refer to the Query Languages piece published on WDVL from XML99 a year ago.

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