XQuery is a query and functional programming language that is designed to query and transform collections of structured and unstructured data, usually in the form of XML, text and with vendor specific extensions other data formats (JSON, binaries, ...).
"The mission of the XML Query project is to provide flexible query facilities to extract data from real and virtual documents on the World Wide Web, therefore finally providing the needed interaction between the Web world and the database world. Ultimately, collections of XML files will be accessed like databases".
XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents.
XQuery contains a superset of uses XPath expression syntax to address specific parts of an XML document.
It supplements this with a SQL-like "FLWOR expression" for performing joins. A FLWOR expression is constructed from the five clauses after which it is named: FOR, LET, WHERE, ORDER BY, RETURN.
The language also provides syntax allowing new XML documents to be constructed. Where the element and attribute names are known in advance, an XML-like syntax can be used; in other cases, expressions referred to as dynamic node constructors are available. All these constructs are defined as expressions within the language, and can be arbitrarily nested.
The language is based on the XQuery and XPath Data Model (XDM) which uses a tree-structured model of the information content of an XML document, containing seven kinds of nodes: document nodes, elements, attributes, text nodes, comments, processing instructions, and namespaces.
XDM also models all values as sequences (a singleton value is considered to be a sequence of length one). The items in a sequence can either be XML nodes or atomic values. Atomic values may be integers, strings, booleans, and so on: the full list of types is based on the primitive types defined in XML Schema.
XQuery 1.0 does not include features for updating XML documents or databases; it also lacks full text search capability. These features are both under active development for a subsequent version of the language. However, the new standards such as XQuery Update Facility 1.0 supports update feature and XQuery and XPath Full Text 1.0 support full text search in XML documents.
XQuery is a programming language that can express arbitrary XML to XML data transformations with the following features:
- Logical/physical data independence
- Declarative
- High level
- Side-effect free
- Strongly typed
What You Should Already Know
Before you continue you should have a basic understanding of the following:
- HTML / XHTML
- XML / XML Namespaces
- XPath
If you want to study these subjects first, find the tutorials on our Home page.
What is XQuery?
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XQuery is About Querying XML
XQuery is a language for finding and extracting elements and attributes from XML documents.
Here is an example of a question that XQuery could solve:
"Select all CD records with a price less than $10 from the CD collection stored in the XML document called cd_catalog.xml"
XQuery and XPath
XQuery 1.0 and XPath 2.0 share the same data model and support the same functions and operators. If you have already studied XPath you will have no problems with understanding XQuery.
XQuery - Examples of Use
XQuery can be used to:
- Extract information to use in a Web Service
- Generate summary reports
- Transform XML data to XHTML
- Search Web documents for relevant information
The XML Example Document
We will use the following XML document in the examples below.
"books.xml":
<?xml version="1.0" encoding="ISO-8859-1"?>
<bookstore>
<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="WEB">
<title lang="en">XQuery Kick Start</title>
<author>James McGovern</author>
<author>Per Bothner</author>
<author>Kurt Cagle</author>
<author>James Linn</author>
<author>Vaidyanathan Nagarajan</author>
<year>2003</year>
<price>49.99</price>
</book>
<book category="WEB">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
</bookstore>
<bookstore>
<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="WEB">
<title lang="en">XQuery Kick Start</title>
<author>James McGovern</author>
<author>Per Bothner</author>
<author>Kurt Cagle</author>
<author>James Linn</author>
<author>Vaidyanathan Nagarajan</author>
<year>2003</year>
<price>49.99</price>
</book>
<book category="WEB">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
</bookstore>
How to Select Nodes From "books.xml"?
Functions
XQuery uses functions to extract data from XML documents.
The doc() function is used to open the "books.xml" file:
doc("books.xml")
Path Expressions
XQuery uses path expressions to navigate through elements in an XML document.
The following path expression is used to select all the title elements in the "books.xml" file:
doc("books.xml")/bookstore/book/title
(/bookstore selects the bookstore element, /book selects all the book elements under the bookstore element, and /title selects all the title elements under each book element)
The XQuery above will extract the following:
<title lang="en">Everyday Italian</title>
<title lang="en">Harry Potter</title>
<title lang="en">XQuery Kick Start</title>
<title lang="en">Learning XML</title>
<title lang="en">Harry Potter</title>
<title lang="en">XQuery Kick Start</title>
<title lang="en">Learning XML</title>
Predicates
XQuery uses predicates to limit the extracted data from XML documents.
The following predicate is used to select all the book elements under the bookstore element that have a price element with a value that is less than 30:
doc("books.xml")/bookstore/book[price<30]
The XQuery above will extract the following:
<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
XQuery Example
for $x in doc("books.xml")/bookstore/book
where $x/price>30
order by $x/title
return $x/title
where $x/price>30
order by $x/title
return $x/title
Thanks http://www.w3schools.com
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