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XQuery

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XQuery is a query language (with some programming language features) that is designed to query collections of XML data. It is semantically similar to SQL.

XQuery 1.0 was developed by the XML Query working group of the W3C. The work was closely coordinated with the development of XSLT 2.0 by the XSL Working Group; the two groups shared responsibility for XPath 2.0, which is a subset of XQuery 1.0. XQuery 1.0 became a W3C Recommendation on January 23, 2007.

"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".[1]


Features

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 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 a tree-structured model of the information content of an XML document, containing seven kinds of node: document nodes, elements, attributes, text nodes, comments, processing instructions, and namespaces.

The type system of the language 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 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, as is done by XUpdate. It also lacks full text search capability. These features are both under active development for a subsequent version of the language.

XQuery is a programming language that can express arbitrary XML to XML data transformations with the following features:

  1. Logical/physical data independence
  2. Declarative
  3. High level
  4. Side-effect free
  5. Strongly typed language

Examples

This example could benefit by including the hamlet.xml so we can see the structure xquery is operating on.

The sample XQuery code below lists the unique speakers in each act of Shakespeare's play Hamlet. 

<html><head/><body>
{
  for $act in doc("hamlet.xml")//ACT
  let $speakers := distinct-values($act//SPEAKER)
  return
    <span>
      <h1>{ $act/TITLE/text() }</h1>
      <ul>
      {
        for $speaker in $speakers
        return <li>{ $speaker }</li>
      }
      </ul>
    </span>
}
</body></html>

XQuery is a functional language consisting entirely of expressions. There are no statements, even though some of the keywords appear to suggest statement-like behaviors. To execute a function, the expression within the body gets evaluated and its value returned. Thus to write a function to double an input value, you simply write: 

declare function local:doubler($x) { $x * 2 }

To write a full query that says Hello World you write the expression: 

"Hello World"

Applications

Below are a few examples of how XQuery can be used:

  1. Extracting information from a database for a use in web service.
  2. Generating summary reports on data stored in an XML database.
  3. Searching textual documents on the Web for relevant information and compiling the results.
  4. Selecting and transforming XML data to XHTML to be published on the Web.
  5. Pulling data from databases to be used for the application integration.
  6. Splitting up an XML document that represents multiple transactions into multiple XML documents.

XQuery and XSLT compared

Although XQuery was initially conceived as a query language for large collections of XML documents, it is also capable of transforming individual documents. As such, its capabilities overlap with XSLT, which was designed expressly to allow input XML documents to be transformed into XML or other formats.

The two standards were developed by separate working groups within W3C, working together to ensure a common approach where appropriate. They share the same data model, type system, and function library, and both include XPath as a sublanguage.

The two languages, however, are rooted in different traditions and serve the needs of different communities. XSLT was primarily conceived as a stylesheet language whose primary goal was to render XML for the human reader on screen, on the web (as web template language), or on paper. XQuery was primarily conceived as a database query language in the tradition of SQL.

Because the two languages originate in different communities, XSLT is stronger in its handling of narrative documents with more flexible structure, while XQuery is stronger in its data handling, for example when performing relational joins.

XSLT 1.0 appeared as a Recommendation in 1999, whereas XQuery 1.0 only became a Recommendation in early 2007; as a result, XSLT is at present much more widely used. Both languages have similar expressive power, though XSLT 2.0 has many features that are missing from XQuery, such as grouping, number and date formatting, and greater control over XML namespaces.[2][3][4]

Any comparison must take into account the fact that XSLT 1.0 and XSLT 2.0 are very different languages. XSLT 2.0, in particular, has been heavily influenced by XQuery in its move to strong typing and schema-awareness.

XQuery is generally considered easier to learn, especially for users with previous experience of database languages such as SQL. This has been confirmed by usability studies.[5] The faster learning curve can be attributed to the fact that XQuery is a smaller language with fewer concepts to learn, and to the fact that programs are more concise. It is also true that XQuery is more orthogonal, in that any expression can be used in any syntactic context. By contrast, XSLT is a two-language system in which XPath expressions can be nested in XSLT instructions but not vice versa.

XSLT is currently stronger than XQuery for applications that involve making small changes to a document (for example, deleting all the NOTE elements). Such applications are generally handled in XSLT by use of a coding pattern that involves an identity template than copies all nodes unchanged, modified by specific templates that modify selected nodes. XQuery has no equivalent to this coding pattern, though in future versions it will be possible to tackle such problems using the update facilities in the language.

Another facility lacking from XQuery is any kind of mechanism for dynamic binding or polymorphism. The absence of this capability starts to become noticeable when writing large applications, or when writing code that is designed to be reusable in different environments. XSLT offers two complementary mechanisms in this area: the dynamic matching of template rules, and the ability to override rules using xsl:import, that make it possible to write applications with multiple customization layers.

The absence of these facilities from XQuery is a deliberate design decision: it has the consequence that XQuery is very amenable to static analysis, which is essential to achieve the level of optimization needed in database query languages. This also makes it easier to detect errors in XQuery code at compile time.

XSLT 2.0 has a number of specific features missing from XQuery 1.0, including grouping constructs, additional constructs for processing text using regular expressions, and formatting of numbers and dates.

The fact that XSLT 2.0 uses XML syntax makes it rather verbose in comparison to XQuery 1.0. However, many large applications take advantage of this capability by using XSLT to read, write, or modify stylesheets dynamically as part of a processing pipeline. The use of XML syntax also enables the use of XML-based tools for managing XSLT code. By contrast, XQuery syntax is more suitable for embedding in traditional programming languages such as Java or C#.

Future work

Currently, two major extensions to the XQuery are under development by W3C:

  • XQuery 1.0 and XPath 2.0 Full-Text [1]
  • XQuery Update facility [2]

Further information

  • Querying XML: XQuery, XPath, and SQL/XML in context. Jim Melton and Stephen Buxton. Morgan Kaufmann, 2006. ISBN 1558607110.
  • XQuery: The XML Query Language. Michael Brundage. Addison-Wesley Professional, 2004. ISBN 0321165810.
  • XQuery from the Experts: A Guide to the W3C XML Query Language. Howard Katz (ed). Addison-Wesley, 2004. ISBN 0-321-18060-7
  • An Introduction to the XQuery FLWOR Expression. Dr. Michael Kay (W3C XQuery Committee), 2005.

Implementations

See also: Category:XQuery processors

References

  1. ^ W3C (2003-10-25). "cited by J.Robie". {{cite web}}: Check date values in: |date= (help)CS1 maint: numeric names: authors list (link)
  2. ^ Kay, Michael (2001-02-03). "Comparing XSLT and XQuery". {{cite web}}: Check date values in: |date= (help)
  3. ^ Eisenberg, J. David (2005-03-09). "Comparing XSLT and XQuery". {{cite web}}: Check date values in: |date= (help)
  4. ^ Smith, Michael (2001-02-23). "XQuery, XSLT "overlap" debated". {{cite web}}: Check date values in: |date= (help)
  5. ^ Usability of XML Query Languages. Joris Graaumans. SIKS Dissertation Series No 2005-16, ISBN 90-393-4065-X

Portions borrowed with permission from the book "XML Hacks" (O'Reilly Media).

Previous version based on an article at the French language Wikipedia.

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