XML Query: what is it, how do I use it and why?

Liam Quin

XML Query: what is it, how do I use it and why?

Liam Quin, W3C

What Is It?

A Query Language for XML
An Integration Language
Syntactic extension to XPath 2
Standardised Middleware

A Query Language for XML

Queries data stored in the XPath and XML Query Data Model
Language uses the XML Schema type system
Construct and manipulate XML fragments as well as regular variables
Results are generally XML (or (X)HTML or text)

Integration Language 2

xquery engine

Integration Language

Integrate and extract data from XML documents, relational data, XML databases and other sources
Query does not need to know how or where data is stored
Move data about without changing the query

Syntactic extension to XPath 2

XQuery syntax is not itself XML
Adds functions...
... modules (function libraries) ...
... new operators ...
... expressions (e.g. FLWOR) ...
and more.

Standardised Middleware

Before XQuery, to connect between Web servers, multiple databases, files, applications, you used...

Standardised Middleware [2]

PROPRIETARY SOFTWARE

Standardised Middleware [3]

PROPRIETARY LANGUAGES

Standardised Middleware [4]

Easier to find staff
reduced trining cost
skill-set part of general XML knowlegde
public documentation
applications easier to maintain
so...

Standardised Middleware [5]

IT’S BETTER.

How do I use it?

Architecture
Implementations
Examples

Architecture [1]

using XQuery with a Web Server

Architecture [2]

copying databases with XQuery/Update

Architecture [3]

Making RDF with XQuery

Architecture [4]

From XML to Printed Page

Architecture [6]

Summary:

XML Query is a good choice for integrating information from a wide range of sources and producing any XML-based vocabulary.

XML Implementations tend to be optimised for handling multiple input sources, and often use indexes for efficiency.

Architecture [7]

IT’S FAST.

Implementations [1]

The XML Query Home Page lists well over forty implementations of XML Query

Implementations [2]

Both open source and proprietary;
In C
In C++
In Haskell
In Java
In vegetables with alfredo sauce
APIs in PHP, Perl, Python, Java, ...

Examples [1]

(: find shirts that fit me :)
let $g := doc("garments.xml"),
for $shirt in $g/garments/item[type = "shirt"]
where
    $shirt/collar = 15
    and not(contains($shirt/style, "stripe"))
return
    <candidate>$shirt/colour</candidate>

Examples [2]

(: find wallpaper images of the Tower of London :)
let $images := doc("jdbc:mysql:localhost:localuser:images"),
	$kw := doc("rdf.xml")
for $item in $rdf/triplets[relation = "locname"]
where
    $item/value = "Tower of London"
    and some $image in $images satisfies (
	$image/uri = $item/uri
	and $image/width = 1024 and $image/height = 768
    )
return
    $item/uri

Examples [2b]

(: find wallpaper images of the Tower of London :)
for $item in $rdf/triplets[relation = "locname"]
where
    $item/value = "Tower of London"
    and some $image in $images satisfies (
	ii:uri($image) = rdf:uri($item)
	and ii:width($image) = 1024
	and ii:height($image) = 768
    )
return
    rdf:uri($item)

Examples [2c]

declare namespace ii = "http://www.example.org/imageinfo/1.6";
declare function ii:width($image as element databaserow)
    as xsi:integer?
{
    return $image/width
};

Why should I use XQuery?

Do things faster and more cheaply
Do things you couldn’t do before
Save money in maintenance fees

Why should I use XQuery? [2]

IT’S COOLER.

Query Status

W3C Candidate Recommendation: no more big changes expected
Widely implemented
Java Community Process standardised on how to call XQuery
ISO SQL standardised how to embed XQuery inside SQL
Good, solid, supported implementations, both open and closed source
IBM DB2, Microsoft SQL Server, Oracle, all include support
Axyana, Galax, Saxonica, eXist: some open source solutions.

Why should I use XQuery? [2]

QUESTIONS?

Questions/Discussion

the scholar looks up from writing

About

Liam Quin,
XML Activity Lead
W3C
liam@w3.org