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XML Query Use Cases: http://www.w3.org/TR/xquery-use-cases/
Copyright ©2003 World Wide Web Consortium, (Massachusetts Institute of Technology, European Research Consortium for Informatics and Mathematics, Keio University). All Rights Reserved. This work is distributed under the W3C® Software License [1] in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
[1] http://www.w3.org/Consortium/Legal/2002/copyright-software-20021231
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 Filename: xmpQ4.xq  
 Source:    http://www.w3.org/TR/xquery-use-cases/#xmp-queries-results-q4 
 Section:   1.1.9.4 Q4  
 Purpose:  For each author in the bibliography, list the author's name and the titles of all  books by that author, grouped inside a "result" element. :)



<results>
  {
    let $a := doc("bib.xml")//author
    for $last in distinct-values($a/last),
        $first in distinct-values($a[last=$last]/first)
    order by $last, $first
    return
        <result>
            <author>
               <last>{ $last }</last>
               <first>{ $first }</first>
            </author>
            {
                for $b in doc("bib.xml")/bib/book
                where some $ba in $b/author 
                      satisfies ($ba/last = $last and $ba/first=$first)
                return $b/title
            }
        </result>
  }
</results>