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

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
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

http://en.wikibooks.org/wiki/XQuery

PB138 — Storing and Querying XML data, XQuery

XQuery

Characteristics

What is XQuery?

XQuery features

XQuery Specification

Processing of Queries

Should we use DOM, XSLT, or XQuery?

Source code example

Example - Simple Query (XPath)

Result

Complex XQuery (FLWOR)

FLWOR — example

Output formatting

Nested queries

XQuery 3

XQuery Update

XQuery Update example

XQuery (XPath) frequently used numeric functions

XQuery (XPath) frequently used element functions

XQuery (XPath) frequently used string functions

Regular expressions in XQuery

User defined functions

Example — function

Quantifiers

Example

Example of if/then/else

For some

Resource on XQuery

Example — `function`

Example of `if/then/else`