XQuery - a technical overview

XQuery
A t e c h n i c a l O v e r v i e w

b y L o r e n C a h l a n d e r
A p r i l 2 3 , 2 0 1 1

XML Query
XQuery is a standardized language for
combining documents, databases, Web pages
and almost anything else. It is very widely
implemented. It is powerful and easy to
learn.

Loren Cahlander - 2011

Functional Programming

Functional programming, and in
particular purely functional
programming, attempts to minimize or
eliminate side effects, and is therefore
considered declarative.

Source: Wikipedia
http://en.wikipedia.org/wiki/Declarative_programming

Two Approaches to
Computation
1940s…

John von Neumann Alonzo Church

Manage state with a program counter. Make functions act like math functions.

Which is simpler? Which will scale to 10,000 CPUs?
Courtesy of Dan McCreary & Associates - 2010
http://www.danmccreary.com/training/xquery/Functional-Programming.pptx

Imperative vs. Functional
Imperative Programming Functional Programming

Focus on a discrete series Focus on independent
of sequential steps that functions that transform
update the state of a data with no regard to
system state management


Different Approaches
Imperative Programming Functional Programming
1.Parameters are data, not 1.Functions are used as
functions parameters to functions
2.Variables are changeable 2.Variables are immutable
3.Use of state to manage 3.Outputs only depend on
calculations the inputs
4.Any object can change 4.No side effects
the state of any other 5.Use of recursion
object at any time
5.Use of sequential steps


Different Thinking
Sequential Processing Parallel Processing

The output of any Each loop of a
step can be used in FLWOR statement is
the next step an independent
thread


Data Model
<p>
<q id="x7">The first q</q>
<q id="x8">The second q</q>
<q href="#x7">The third q</q>
</p>


XML Schema

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>

Built In Functions
Handling XML Schema datatypes
Used by XPath 2.0 and XQuery 1.0
Accessors Functions and Operators on Durations, Dates and Times

The Error Function Functions Related to QNames

The Trace Function Operators on base64Binary and hexBinary

Constructor Functions Operators on NOTATION

Functions and Operators on Numerics Functions and Operators on Nodes

Functions on Strings Functions and Operators on Sequences

Functions on anyURI Context Functions

Functions and Operators on Boolean Values


Path Expressions
Find the ﬁrst section of the chapter with the title
“Starry Night” of the document named “alpha.xml”:

document(“alpha.xml”)//chapter[title = “Starry Night”]/section[1]


Element Constructors

let $url-params :=
<params>
<param name="web-path-to-site">{$web-path-to-site}</param>
<param name="web-path-to-app">{$web-path-to-app}</param>
<param name="db-path-to-site">{$db-path-to-site}</param>
<param name="db-path-to-app">{$db-path-to-app}</param>
<param name="db-path-to-app-data">{$db-path-to-app-data}</param>
<param name="web-depth-in-site">{$web-depth-in-site}</param>
</params>


FLWORPronounced FLOWER

for var in expression
For and let clauses generate a list of
at posvar
variable bindings preserving document
, order.
let var := expression
Where applies a predicate, eliminating
some of the bindings. It is more
efficient to apply a predicate in the
where expression XPath expression before using a where
clause.
Order by changes the document order
,
based on the expression
ascending Return is executed for each surviving
order by expression
binding, generating an ordered list of
descending
output.

return expression


Functions
(:~
URL Rewriting Module
Contains functions for all URL Rewriting needs across the entire site
:)

module namespace url = "http://exist-db.org/ns/url-rewriting/1.0";

declare function url:log-variables($params as element(params)) {
for $param in $params/param
let $name := string($param/@name)
let $value := $param/text()
return
util:log("DEBUG", concat("URL Rewriter: ", $name, ": ", $value))
};


Conditionals
declare function dates:_processYear($year as xs:string?) as xs:string?
{
! if($year castable as xs:integer)
! then
! ! if(xs:integer($year) < 100)
! ! then
! ! ! if(xs:integer($year) > 50)
! ! ! then concat("19", xs:string(xs:integer($year)))
! ! ! else if(xs:integer($year) = 0)
! ! ! then "1900"
! ! ! else concat("20", string-pad("0", 2 - string-length($year)), $year)
! ! else if(xs:integer($year) >= 100 and xs:integer($year) < 200) (: 102 = 2002 in Java :)
! ! then xs:string(1900 + xs:integer($year))
! ! else if(xs:integer($year) > 9999)
! ! then "9999"
! ! else $year
! else $year
};


Examples

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


XQuery Wikibook
FLWOR Expression Example

Motivation
You have a sequence of items and you want to create a report that contains these items.

Method
We will use a basic XQuery FLWOR expression to iterate through each of the items in a sequence. The ﬁve parts of a FLWOR expression are:
■ for - speciﬁes what items in the sequence you want to select (optional)
■ let - used to create temporary names used in the return (optional)
■ where - limit items returned (optional)
■ order - change the order of the results (optional)
■ return - specify the structure of the data returned (required)
Here is a simple example of a FLWOR expression:
for $book in doc("catalog.xml")/books/book
let $title := $book/title/text()
let $price := $book/price/text()
where xs:decimal($price) gt 50.00
order by $title
return
<book>
<title>{$title}</title>
<price>{$price}</price>
</book>

This XQuery FLWOR expression will return all books that have a price over $50.00. Note that we have not just one but two let statements after the for loop.
We also add a where clauses to restrict the results to books over $50.00. The results are sorted by the title and the result is a new sequence of book items
with both the price and the title in them.

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

XQDoc
Documentation Generation

(:~
: The controller for constructing the xqDoc HTML information for
: the specified library module. The following information for
: each library module will be generated.
: <ul>
: <li> Module introductory information</li>
: <li> Global variables declared in this module</li>
: <li> Modules imported by this module</li>
: <li> Summary information for each function defined in the module</li>
: <li> Detailed information for each function defined in the module</li>
: </ul>
:
: @param $uri the URI for the library module
: @param $local indicates whether to build static HTML link for offline
: viewing or dynamic links for real-time viewing.
: @return XHTML.
:)
define function print-module($uri as xs:string, $local as xs:boolean) as element()*


XML Query Test Suite

The XML Query Test Suite (XQTS) is to show that the
actual speciﬁcation can be implemented as written, and
to demonstrate interoperability. W3C does not test or
certify implementations.


Implementations

SAXON

Qexo

http://www.w3.org/XML/Query/#implementations

EXPath/EXQuery
EXQuery
(:~ Collaboratively Defining Open Standards for Portable XQuery Applications :)

EXQuery will define standards for -
● Extending XQuery for Application Development.
● Portable XQuery Applications.

EXQuery is -
● Open – everything is transparent and visible.
● Collaborative – anyone may join us.

● Community driven – standards from XQuery Application Developers.

...launching TODAY at XML Prague 2009 :-)

www.exquery.org
XML Prague 2009 Poster Session
Adam Retter
adam.retter@exquery.org

XQuery 3.0
W3C Working Draft

group by clause in FLWOR Expressions
tumbling window and sliding window in FLWOR Expressions
count clause in FLWOR Expressions
allowing empty in 3.9.2 For Clause, for functionality similar to outer joins in SQL.
try/catch expressions
Dynamic function invocation
Inline functions
Private functions
Nondeterministic functions
Switch expressions
Computed namespace constructors
Output declarations
Annotations
Annotation assertions in function tests


Documentation
XQuery 1.0: An XML Query Language (W3C Recommendation)
http://www.w3.org/TR/xquery/
XML Query (XQuery) 1.0 Requirements (W3C Working Group Note)
http://www.w3.org/TR/xquery-requirements/
XML Query 1.0 Use Cases (W3C Working Group Note)
http://www.w3.org/TR/xquery-use-cases/
Building a Tokenizer for XPath or XQuery (Joint Note)
http://www.w3.org/TR/xquery-xpath-parsing/
XQuery 3.0: An XML Query Language (W3C Working Draft)
http://www.w3.org/TR/xquery-30/

XQuery
xqDoc
http://xqdoc.org/index.html
Search Across a Variety of XML Data
By Priscilla Walmsley XML Query Test Suite
Publisher: O'Reilly Media
Released: March 2007 http://dev.w3.org/2006/xquery-test-suite/PublicPagesStagingArea/
EXPath/EXQuery
http://www.expath.org/
http://www.exquery.org/


XQuery - a technical overview

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