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

XQuery Specification

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

  • 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.

 <?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:

<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.

  • (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.

  • (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>

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

  • 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) frequently used 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.

  • 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) frequently used 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.

  • 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.

  • 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) frequently used 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

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

<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>

For some

Resource on XQuery