XQuery

XQuery
Paradigmdeclarative, functional, modular
Designed byW3C
First appeared2007
3.1 / March 21, 2017; 2 years ago (2017-03-21)[1]
Typing disciplinedynamic or static,[2][3] strong
OSCross-platform
www.w3.org/XML/Query/
Major Many
Influenced by
XPath, SQL, XSLT

XQuery (XML Query) is a query and functional programming language that queries and transforms collections of structured and unstructured data, usually in the form of XML, text and with vendor-specific extensions for other data formats (JSON, binary, etc.). The language is developed by the XML Query working group of the W3C. The work is closely coordinated with the development of XSLT by the XSL Working Group; the two groups share responsibility for XPath, which is a subset of XQuery.

XQuery 1.0 became a W3C Recommendation on January 23, 2007.[4]

XQuery 3.0 became a W3C Recommendation on April 8, 2014.[5]

XQuery 3.1 became a W3C Recommendation on March 21, 2017.[6]

"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".[7]

Features

XQuery is a functional, side effect-free, expression-oriented programming language with a simple type system, summed up by Kilpeläinen:[8]

All XQuery expressions operate on sequences, and evaluate to sequences. Sequences are ordered lists of items. Items can be either nodes, which represent components of XML documents, or atomic values, which are instances of XML Schema base types like xs:integer or xs:string. Sequences can also be empty, or consist of a single item only. No distinction is made between a single item and a singleton sequence. (...) XQuery/XPath sequences differ from lists in languages like Lisp and Prolog by excluding nested sequences. Designers of XQuery may have considered nested sequences unnecessary for the manipulation of document contents. Nesting, or hierarchy of document structures is instead represented by nodes and their child-parent relationships

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[9] or office documents.

XQuery contains a superset of 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.

Features for updating XML documents or databases, and full text search capability, are not part of the core language, but are defined in add-on extension standards: XQuery Update Facility 1.0 supports update feature and XQuery and XPath Full Text 1.0 supports full text search in XML documents.

XQuery 3.0 adds support for full functional programming, in that functions are values that can be manipulated (stored in variables, passed to higher-order functions, and dynamically called).