PB138 — Storing and Querying XML
data, XQuery
XQuery
• is a query language for searching and extraction of XML nodes (elements, attributes) from a
document and for an output XML document construction.
• Created by a different W3C group (community) than XSLT
• Purpose might be the same but XQuery tends to be used for more structured data while XSLT
rather for documents (which are more narrative, less structured).
• SQL-like
Characteristics
• The XQuery is the most common XML query language at present time (and it seems to be in
future as well).
• Based on XPath 2.0 data model (XQuery 1.0) or common XML Schema and XPath 3.0 data model,
operators and functions in case of XQuery 3.0.
• Supported by main database engines producers (IBM, MS, Oracle, etc)
What is XQuery?
• Combines XPath 2.0
• FLWOR construct(s) — main backbone of the language
• Element constructors
• User-defined functions
• other directives
XQuery features
• All XQuery expressions operate on sequences, and evaluate to sequences, i.e.
• list of nodes or atomic values
• Extending XPath, adding complex queries
• Extension for updates
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XQuery Specification
• Developed by W3C Consortium — XML Query Group in collaboration with the XSL Working
Group
• XQuery 1.0 became a W3C Recommendation on 14 December 2010
• XQuery 3.0 became a W3C Recommendation on April 8, 2014
• The XQuery timeline
Processing of Queries
• Native XML databases, such as BaseX — used for "real" querying within collections of XML
document
• XML-enabled databases
• Some XSLT/XQuery processors, such as Saxon — usually for querying just one document
Should we use DOM, XSLT, or XQuery?
• Tasks where extraction (selection) part is more complicated than the construction part →
use XQuery
Should we use DOM, XSLT, or XQuery?
• In other cases, i.e. more complex output is required →
◦ if more "narrative", less structured input → use XSLT
◦ if more complex operations are required → use a more general API such as DOM
Source code example
Example of source document, XML Queries on it and their results.
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 <?xml version="1.0" encoding="UTF-8"?>
  <addressbook>
  <person category="friends">
  <firstname>Petr</firstname>
  <lastname>Novak</lastname>
  <date-of-birth>1969-05-14</date-of-birth>
  <email>novak@myfriends.com</email>
  <characteristics lang="en">Very good friend</characteristics>
  </person>
  <person category="friends">
  <firstname>Jaroslav</firstname>
  <lastname>Nováček</lastname>
  <date-of-birth>1968-06-14</date-of-birth>
  <email>novacek@myfriends.com</email>
  <characteristics lang="en">Another good friend</characteristics>
  </person>
  <person category="staff">
  <firstname>Jan</firstname>
  <lastname>Horak</lastname>
  <date-of-birth>1970-02-0</date-of-birth>
  <email>horak@mycompany.com</email>
  <characteristics lang="en">Just colleague</characteristics>
  </person>
  <person category="friends">
  <firstname>Erich</firstname>
  <lastname>Polak</lastname>
  <date-of-birth>1980-02-28</date-of-birth>
  <email>erich@myfriends.com</email>
  <characteristics lang="en">Good friend</characteristics>
  </person>
  </addressbook>
Example - Simple Query (XPath)
• Task: "extract all surnames in the addressbook".
• Query is more-or-less just an XPath expression, like "selects all lastname elements":
doc('myaddresses.xml')/addressbook/person/lastname
Result
The query to above mentioned document will output:
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<lastname>Novák</lastname> <lastname>Nováček</lastname>
<lastname>Horák</lastname> <lastname>Polák</lastname>
Complex XQuery (FLWOR)
FLWOR is an acronym of an XQuery structure. It roughly corresponds to the SQL query structure:
• (F)or — Initial query part that specifies query cycle including control variable. Results of XPath
expression behind the keyword " in" are assigned to the variable.
• (L)et — You can assign values of next variable that can be used later in this section.
Complex XQuery (FLWOR)
FLWOR is an acronym of an XQuery structure. It roughly corresponds to the SQL query structure:
• (W)here — specifies selection condition ie. which nodes (values) selected by for section will be
used.The condition can utilize the variables defined in the "let" section.
Complex XQuery (FLWOR)
FLWOR is an acronym of an XQuery structure. It roughly corresponds to the SQL query structure:
• (O)rder — Defines how the nodes should be oredered.
• Return — Defines what is returned, constructed from extracted nodes (values).
FLWOR — example
• Condition used to select requested nodes can be specified either in an XPath expression in for
clause or in the where clause.
"Return Mr. Polak’s birth-date." →
for $person in doc('myaddresses.xml')/addressbook/person
where $person/lastname='Polák'
return $person/date-of-birth
XQuery returns →
<?xml version=" 1.0" encodings"UTF-8"?>
<date-of-birth>l980-02-28</date-of-birth>
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Output formatting
• Return clause may contain XML tags, creating new output XML document
• Enclosed expression to evaluate in { }
for $person in doc('myaddresses.xml')/addressbook/person
where $person/lastname='Polák'
return <datum>{$person/date-of-birth/text()}</datum>
XQuery returns →
<datum>1980-02-28</datum>
Nested queries
• Queries may be nested in each other, use { }
for $person in doc('myaddresses.xml')/addressbook/person
where $person/lastname='Polák'
return
  <osoba>
  <chars>
  {for $char in $person/characteristics return <char>{$char/text()}</char>}
  </chars>
  </osoba>
XQuery returns →
<osoba>
  <chars>
  <char>Good friend</char>
  </chars>
</osoba>
XQuery 3
• group by
• less strict order of elements
• function items (lambda functions)
• let $f := function($x, $y) { $x + $y } return $f(17, 25)
• try/catch
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• switch
• …
XQuery Update
• XQuery Update Facility 1.0, Recommendation 17 March 2011
• Extension to provide update features
• Insert nodes
• Delete nodes
• Replace value/node
XQuery Update example
update insert <email>new@mail.com</email> into
for $person in doc('myaddresses.xml')/addressbook/person
where $person/lastname='Polák'
update delete doc('myaddresses.xml')/addressbook/person/lastname
XQuery (XPath) numeric functions
• count($seq as item()*) Counts the items in a sequence.
• sum($seq as item()*) Returns the sum of the items in a sequence.
• avg($seq as item()*) Returns the average of the items in a sequence.
XQuery (XPath) numeric functions
• min($seq as item()*) Returns the minimum valued item in a sequence.
• max($seq as item()*) Returns the maximum valued item in a sequence.
XQuery (XPath) element functions
• distinct-values($seq as item()*) Returns select distinct items from a sequence.
• subsequence($seq as item()*, $startingLoc as xs:double, $length as xs:double) Returns a
subset of provided sequence.
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XQuery (XPath) element functions
• insert-before($seq as item(), $position as xs:integer, $inserts as item()) Inserts an item in
a sequence.
• remove($seq as item()*, $position as xs:integer) Removes an item from a sequence.
• reverse($seq as item()*) Returns the reversed sequence.
XQuery (XPath) element functions
• index-of($seq as anyAtomicType()*, $target as anyAtomicType()) Returns indexes as integers to
indicate availability of an item within a sequence.
• last() Returns the last element of a sequence when used in predicate expression.
• position() Used in FLOWR expressions to get the position of an item in a sequence.
XQuery (XPath) string functions
• string-length($string as xs:string) as xs:integer Returns the length of the string.
• concat($input as xs:anyAtomicType?) as xs:string Returns the concatenated string as output.
• string-join($sequence as xs:string*, $delimiter as xs:string) as xs:string Returns the
combination of items in a sequence separated by a delimiter.
Regular expressions in XQuery
• matches($input, $regex) Returns true if the input matches with the provided regular expression.
• replace($input, $regex, $string) Replaces the matched input string with given string.
• tokenize($input, $regex) Returns a sequence of items matching the regular expression.
User defined functions
• allow to create a named function (possibly with parameters) to be used in query script
• can be assigned to a "local" or other namespace
Example — function
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declare namespace company='http://www.mycompany.com';
declare function company:first_person() as node() {
  <person id="1">
  <firstname>Tomáš</firstname>
  <surname>Novák</surname>
  </person>
};
Quantifiers
• Enable to construct conditions testing whether some condition is satisfied "for all" or "for some"
items
• some
• all
Example
• returns those element from the first list that are also present in the second list
• syntax: some item in list satisfies condition
declare function local:in_both_lists($list1 as node()*, $list2 as node()*) as
node()* {
  for $item1 in $list1/item
  let $item-text := $item1/text()
  return
  if (some $item2 in $list2/item satisfies $item2/text() = $item1/text())
  then $item1
  else ()
};
Example of if/then/else
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<result>
{
  if(not(doc("books.xml"))) then (
  <error>
  <message>books.xml does not exist</message>
  </error>
  )
  else (
  for $x in doc("books.xml")/books/book
  where $x/price>30
  return $x/title
  )
}
</result>
Resource on XQuery
• http://en.wikibooks.org/wiki/XQuery
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