(C) 2021 Masaryk University -- T. Pitner, L. Bártek, L. Grolig, A. Rambousek
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
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)
Combines XPath 2.0
FLWOR construct(s) — main backbone of the language
Element constructors
User-defined functions
other directives
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
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
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
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>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/lastnameThe query to above mentioned document will output:
<lastname>Novák</lastname> <lastname>Nováček</lastname>
<lastname>Horák</lastname> <lastname>Polák</lastname>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.
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.
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).
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-birthXQuery returns →
<?xml version=" 1.0" encodings"UTF-8"?>
<date-of-birth>l980-02-28</date-of-birth>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>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>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 Facility 1.0, Recommendation 17 March 2011
Extension to provide update features
Insert nodes
Delete nodes
Replace value/node
update insert <email>new@mail.com</email> into
for $person in doc('myaddresses.xml')/addressbook/person
where $person/lastname='Polák'update delete doc('myaddresses.xml')/addressbook/person/lastnamecount($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.
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()*, $target as anyAtomicType()) Returns indexes as integers to indicate availability of an item within a sequence.
last() Returns the last element of a sequence when used in predicate expression.
position() Used in FLOWR expressions to get the position of an item in a sequence.
string-length($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*, $delimiter as xs:string) as xs:string Returns the combination of items in a sequence separated by a delimiter.
matches($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, $regex) Returns a sequence of items matching the regular expression.
allow to create a named function (possibly with parameters) to be used in query script
can be assigned to a "local" or other namespace
functiondeclare namespace company='http://www.mycompany.com';
declare function company:first_person() as node() {
<person id="1">
<firstname>Tomáš</firstname>
<surname>Novák</surname>
</person>
};Enable to construct conditions testing whether some condition is satisfied "for all" or "for some" items
some
all
returns those element from the first list that are also present in the second list
syntax: some item in list satisfies condition
declare function local:in_both_lists($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 ()
};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>