Introduction to XQuery
XQuery is about extracting information from XML documents.
What You Should Already Know
Before you study XQuery you should have a basic understanding of XML and XML Namespaces.
If you want to study these subjects first, please read our XML Tutorial.
What is XQuery?
XQuery is a language for querying XML data
XQuery is built on XPath expressions
XQuery for XML is like SQL for databases
XQuery is About Querying XML
XQuery is a language for finding and extracting (querying) data 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 shares the same data model, the same functions, and the same syntax.
If you have already studied XPath you will have no problems with understanding XQuery.
You can read more about XPath in our XPath Tutorial.
XQuery is Not (yet) a Web Standard
XQuery 1.0 is still a W3C Working Draft.
It is expected to reach a W3C Recommendation status early in 2004.
To read more about the XQuery activity at W3C, please read our W3C Tutorial.
XQuery Example
Let's try to learn some XQuery by an example.
The XML Example Document
We will use this XML document called "books.xml" in all our examples:
<?xml version="1.0" encoding="ISO-8859-1"?>
<bib>
<book year="1994">
<title>TCP/IP Illustrated</title>
<author><last>Stevens</last><first>W.</first></author>
<publisher>Addison-Wesley</publisher>
<price>65.95</price>
</book>
<book year="1992">
<title>Advanced Programming in the Unix environment</title>
<author><last>Stevens</last><first>W.</first></author>
<publisher>Addison-Wesley</publisher>
<price>65.95</price>
</book>
<book year="2000">
<title>Data on the Web</title>
<author><last>Abiteboul</last><first>Serge</first></author>
<author><last>Buneman</last><first>Peter</first></author>
<author><last>Suciu</last><first>Dan</first></author>
<publisher>Morgan Kaufmann Publishers</publisher>
<price>39.95</price>
</book>
<book year="1999">
<title>The Technology and Content for Digital TV</title>
<editor>
<last>Gerbarg</last><first>Darcy</first>
<affiliation>CITI</affiliation>
</editor>
<publisher>Kluwer Academic Publishers</publisher>
<price>129.95</price>
</book>
</bib>
Click here to view the XML file in your browser
Extracting Nodes With a Functions
XQuery uses functions to extract data from XML documents.
The following XQuery:
doc("books.xml")
Will extract the full XML document from the file "books.xml". (In exact phrases the function returns the document node of the document)
Extracting Nodes With a Node Path
XQuery uses XPath node paths to extract data from XML documents.
The following XQuery :
doc("books.xml")/bib/book/title
Will extract the following data:
<title>TCP/IP Illustrated</title>
<title>Advanced Programming in the Unix environment</title>
<title>Data on the Web</title>
<title>The Technology and Content for Digital TV</title>
In the example above, the function doc("book.xml") is used to open the XML document. (In exact phrases the function returns the document node of the document)
The XPath node path /bib/book/title is used to extract all the title elements. (/bib selects the bib element, /book selects all the book elements under the bib element, and /title selects all the title elements under each book element)
If you know how XPath works, you will know that this XQuery will give the same result:
doc("books.xml")//title
Extracting Nodes With an Expression
XQuery uses expressions to extract data from XML documents.
The following XQuery :
doc("books.xml")/bib/book[price<50]
Will extract the following data:
<book year="2000">
<title>Data on the Web</title>
<author><last>Abiteboul</last><first>Serge</first></author>
<author><last>Buneman</last><first>Peter</first></author>
<author><last>Suciu</last><first>Dan</first></author>
<publisher>Morgan Kaufmann Publishers</publisher>
<price>39.95</price>
</book>
In the example above, the function doc("book.xml") returns the document node of the document.
The XPath node path /bib/book selects all the book elements under the bib element, and the expression (condition) [price<50] selects a subset of the book elements. (Only the book elements with a price element with a value less than 50)
XQuery FLWOR
FLWOR = For, Let, Where, Order by, Return.
The XML Example Document
We will use this XML document called "books.xml" in all our examples:
<?xml version="1.0" encoding="ISO-8859-1"?>
<bib>
<book year="1994">
<title>TCP/IP Illustrated</title>
<author><last>Stevens</last><first>W.</first></author>
<publisher>Addison-Wesley</publisher>
<price>65.95</price>
</book>
<book year="1992">
<title>Advanced Programming in the Unix environment</title>
<author><last>Stevens</last><first>W.</first></author>
<publisher>Addison-Wesley</publisher>
<price>65.95</price>
</book>
<book year="2000">
<title>Data on the Web</title>
<author><last>Abiteboul</last><first>Serge</first></author>
<author><last>Buneman</last><first>Peter</first></author>
<author><last>Suciu</last><first>Dan</first></author>
<publisher>Morgan Kaufmann Publishers</publisher>
<price>39.95</price>
</book>
<book year="1999">
<title>The Technology and Content for Digital TV</title>
<editor>
<last>Gerbarg</last><first>Darcy</first>
<affiliation>CITI</affiliation>
</editor>
<publisher>Kluwer Academic Publishers</publisher>
<price>129.95</price>
</book>
</bib>
Click here to view the XML file in your browser
Extracting Nodes With FLWOR
The following FLWOR expression:
for $x in doc("books.xml")/bib/book
where $x/price>50
order by $x/title
return $x/title
Will extract the following data:
<title>Advanced Programming in the Unix environment</title>
<title>TCP/IP Illustrated</title>
<title>The Technology and Content for Digital TV</title>
In the example above:
The for clause selects all book nodes (book elements under the bib element) into a variable called $x.
The where clause selects only the $x nodes (book nodes) with price elements with a value greater than 50.
The order by clause orders the $x nodes (book nodes) by the value of the title elements (title nodes).
The return clause returns the title nodes.
