XQuery
Encyclopedia
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 databaseRelational database
A relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...
s or office documents.
XQuery uses XPath
XPath
XPath is a language for selecting nodes from an XML document. In addition, XPath may be used to compute values from the content of an XML document...
expression syntax to address specific parts of an XML document. It supplements this with a SQL
SQL
SQL is a programming language designed for managing data in relational database management systems ....
-like "FLWOR
FLWOR
The programming language XQuery defines FLWOR as an expression that supports iteration and binding of variables to intermediate results. FLWOR is an acronym: FOR, LET, WHERE, ORDER BY, RETURN...
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; it also lacks full text search
Full text search
In text retrieval, full text search refers to techniques for searching a single computer-stored document or a collection in a full text database...
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
Examples
The sample XQuery code below lists the unique speakers in each act of Shakespeare's play Hamlet, encoded in hamlet.xml{
for $act in doc("hamlet.xml")//ACT
let $speakers := distinct-values($act//SPEAKER)
return
}
All XQuery constructs for performing computations are expressions
Expression (programming)
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces another value...
. There are no statements
Statement (programming)
In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program written in such a language is formed by a sequence of one or more statements. A statement will have internal components .Many languages In computer programming...
, even though some of the keywords appear to suggest statement-like behaviors. To execute a function, the expression within the body is evaluated and its value is returned. Thus to write a function to double an input value, one simply writes:
declare function local:doubler($x) { $x * 2 }
To write a full query saying 'Hello World', one writes the expression:
"Hello World"
This style is common in functional programming languages
Functional programming
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
. However, unlike most functional programming languages, XQuery 1.0 doesn't support higher-order function
Higher-order function
In mathematics and computer science, higher-order functions, functional forms, or functionals are functions which do at least one of the following:*take one or more functions as an input*output a function...
s (they first appear in the drafts for XQuery 3.0).
Applications
Below are a few examples of how XQuery can be used:- Extracting information from a database for use in a web service.
- Generating summary reports on data stored in an XML database.
- Searching textual documents on the Web for relevant information and compiling the results.
- Selecting and transforming XML data to XHTML to be published on the Web.
- Pulling data from databases to be used for the application integration.
- Splitting up an XML document that represents multiple transactions into multiple XML documents.
Scope
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 XSLTXSLT
XSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
, which was designed expressly to allow input XML documents to be transformed into HTML or other formats.
The XSLT 2.0 and XQuery 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
XPath
XPath is a language for selecting nodes from an XML document. In addition, XPath may be used to compute values from the content of an XML document...
2.0 as a sublanguage.
Origin
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 languageWeb template
A web template is a tool used to separate content from presentation in web design, and for mass-production of web documents. It is a basic component of a web template system.Web templates can be used to set up any type of website...
), or on paper. XQuery was primarily conceived as a database query language in the tradition of SQL
SQL
SQL is a programming language designed for managing data in relational database management systems ....
.
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).
Versions
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 1.0, such as grouping, number and date formatting, and greater control over XML namespaces. Many of these features are planned for XQuery 3.0.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.
Strengths and weaknesses
Usability studies have shown that XQuery is easier to learn than XSLT, especially for users with previous experience of database languages such as SQL. This 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 that 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 that are under development.
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.
The fact that XSLT 2.0 uses XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
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
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
or C#. If necessary, XQuery code can also be expressed in an XML syntax called XQueryX. The XQueryX representation of XQuery code is rather verbose and not convenient for humans, but can easily be processed with XML tools, for example transformed with XSLT stylesheets.
Future work
Currently, two major extensions to the XQuery are under development by W3C:- XQuery 1.0 and XPath 2.0 Full-Text
- XQuery Update FacilityXQuery Update FacilityXQuery Update Facility is an extension to the XML Query language, XQuery. It provides expressions that can be used to make changes to instances of the XQuery 1.0 and XPath 2.0 Data Model....
Work has started on XQuery 3.0 (formerly: 1.1). Planned new features are listed in a new requirements document.
Scripting (procedural) extension for XQuery is also being designed.
Further reading
- Querying XML: XQuery, XPath, and SQL/XML in context. Jim Melton and Stephen Buxton. Morgan Kaufmann, 2006. ISBN 1558607110.
- XQuery. Priscilla Walmsley. O'Reilly Media, 2007. ISBN 0596006349.
- 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 KayMichael Kay (software engineer)Michael H. Kay FBCS is the editor of the W3C specification of the XSLT 2.0 language for performing XML transformations, and the developer of the Saxon XSLT and XQuery processing software....
(W3C XQuery Committee), 2005.
Implementations
External links
- W3C XML Query (XQuery)
- XQuery tutorial
- XQuery API for JavaXQuery API for JavaXQuery API for Java refers to the common Java API for the W3C XQuery 1.0 specification. It was submitted under the Java Community Process as JSR 225 , and final release was made available on June 24, 2009.The XQJ API enables a Java developer to:...
(XQJ) Java Specification Request - hamlet.xml Hamlet in XML Format
- XQuery (presentation - as HTML slides)
- FunctX XQuery Functions - open sourceOpen sourceThe term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
pure XQuery function library, tested with Saxon - Discovering XQuery Blog
- XQuery Development Tools for Eclipse (XQDT)
- BaseX: XQuery Update/Full Text implementation with realtime XQuery editor; Live Demo.
- XQuery Live Demo of the FLWOR-Foundation
Portions borrowed with permission from the books "XML Hacks" (O'Reilly Media
O'Reilly Media
O'Reilly Media is an American media company established by Tim O'Reilly that publishes books and Web sites and produces conferences on computer technology topics...
) and "XQuery" (O'Reilly Media
O'Reilly Media
O'Reilly Media is an American media company established by Tim O'Reilly that publishes books and Web sites and produces conferences on computer technology topics...
).
Previous version based on an article at the French language Wikipedia