Storing and Querying XML data; XQuery

XQuery

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

XQuery features

All XQuery expressions operate on sequences, and evaluate to sequences
List of Nodes or atomic values
Extending XPath, adding complex queries
Extension for updates

Specifications

The XQuery is the most common XML query language at present time (and it seems to be the same in the 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)

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

Where XQuery and where XSLT?

The XQuery domain is useful for:

queries, where extraction (selection) part is more complicated than the construction part.

In other cases:

Use XSLT for more "narrative", less structured inputs and if more complex output is required (e.g. added some new markup and content).
Otherwise use a more general API (using DOM manipulation) if more complex operations are required.

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.
®eturn: 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

Resource on XQuery

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