About XQuery

XML is a tree-like data structure with seven kinds of nodes (document, element, attribute, text, comment, processing instruction, namespace). The XQuery data model has these seven node and 50 “atomic” types as well. The 50 may be arranged under the headings of untyped, Boolean, numeric, string, calendar, qualified name, and other types.

Documents are interrogated using XPath expressions. (See the Wikipedia entry for XPath.)

It is very easy to construct XML using XQuery.

In XQuery, all input and output must be an instance of the XQuery data model. Every instance is a sequence of zero or more items, and every item is one of the seven node or 50 atomic types.

Some of you are old enough to remember the “GOTO” statement, which became extinct because of an article by Edsger Dijkstra called “A Case against the GO TO Statement” (1968). The 1977 Turing Award lecture by John Backus (who invented FORTRAN and BNF) entitled Can Programming Be Liberated From the von Neumann Style? may send the assignment statement the same way. [Kay 2004, 625-9]

What!? How then do I program? In order to become functional, you are best to forget everything you know about procedural programming. Functional programs are like that frightful mathematics you learned at university with statements such as “for every,” “there exists,” “let N be the set of all positive integers.” Functions do everything interesting. Side-effects are forbidden (i.e. existing values are not clobbered). If the procedural left-side of your brain tells you to use iteration, the functional right-side must come up with a recursive solution instead.

Advantages of this approach are:

The original XML document can be transformed but not updated.

One can only apply pattern matching to individual text nodes. A program is necessary to look through the whole document.

<bib>
    
    <book year="1994">
        <title>TCP/IP Illustrated</title>
        <author><last>Stevens</last><first>W.</first></author>
        <publisher>Addison-Wesley</publisher>
        <price>65.95</price>
    </book>
 
    <book year="1992">
        <title>Advanced Programming in the Unix environment</title>
        <author><last>Stevens</last><first>W.</first></author>
        <publisher>Addison-Wesley</publisher>
        <price>65.95</price>
    </book>
 
    <book year="2000">
        <title>Data on the Web</title>
        <author><last>Abiteboul</last><first>Serge</first></author>
        <author><last>Buneman</last><first>Peter</first></author>
        <author><last>Suciu</last><first>Dan</first></author>
        <publisher>Morgan Kaufmann Publishers</publisher>
        <price>39.95</price>
    </book>
 
    <book year="1999">
        <title>The Economics of Technology and Content for Digital TV</title>
        <editor>
               <last>Gerbarg</last><first>Darcy</first>
                <affiliation>CITI</affiliation>
        </editor>
            <publisher>Kluwer Academic Publishers</publisher>
        <price>129.95</price>
    </book>
 
</bib>

    

<bib>
 {
  for $b in doc("http://bstore1.example.com/bib.xml")/bib/book
  where $b/publisher = "Addison-Wesley" and $b/@year > 1991
  return
    <book year="{ $b/@year }">
     { $b/title }
    </book>
 }
</bib>

          

<bib>
    <book year="1994">
        <title>TCP/IP Illustrated</title>
    </book>
    <book year="1992">
        <title>Advanced Programming in the Unix environment</title>
    </book>
</bib>

          

<results>
  {
    let $a := doc("http://bstore1.example.com/bib/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("http://bstore1.example.com/bib.xml")/bib/book
                where some $ba in $b/author 
                      satisfies ($ba/last = $last and $ba/first=$first)
                return $b/title
            }
        </result>
  }
</results>

          

<results>
    <result>
        <author>
            <last>Abiteboul</last>
            <first>Serge</first>
        </author>
        <title>Data on the Web</title>
    </result>
    <result>
        <author>
            <last>Buneman</last>
            <first>Peter</first>
        </author>
        <title>Data on the Web</title>
    </result>
    <result>
        <author>
            <last>Stevens</last>
            <first>W.</first>
        </author>
        <title>TCP/IP Illustrated</title>
        <title>Advanced Programming in the Unix environment</title>
    </result>
    <result>
        <author>
            <last>Suciu</last>
            <first>Dan</first>
        </author>
        <title>Data on the Web</title>
    </result>
</results>

          

<bib>
  {
    for $b in doc("http://bstore1.example.com/bib.xml")//book
    where count($b/author) > 0
    return
        <book>
            { $b/title }
            {
                for $a in $b/author[position()<=2]  
                return $a
            }
            {
                if (count($b/author) > 2)
                 then <et-al/>
                 else ()
            }
        </book>
  }
</bib>
          

<bib>
    <book>
        <title>TCP/IP Illustrated</title>
        <author>
            <last>Stevens</last>
            <first>W.</first>
        </author>
    </book>
    <book>
        <title>Advanced Programming in the Unix environment</title>
        <author>
            <last>Stevens</last>
            <first>W.</first>
        </author>
    </book>
    <book>
        <title>Data on the Web</title>
        <author>
            <last>Abiteboul</last>
            <first>Serge</first>
        </author>
        <author>
            <last>Buneman</last>
            <first>Peter</first>
        </author>
        <et-al/>
    </book>
</bib>