XQuery: An XML query language

The World Wide Web Consortium has convened a working group to design a query language for Extensible Markup Language (XML) data sources. This new query language, called XQuery, is still evolving and has been described in a series of drafts published by the working group. XQuery is a functional language comprised of several kinds of expressions that can be nested and composed with full generality. It is based on the type system of XML Schema and is designed to be compatible with other XML-related standards. This paper explains the need for an XML query language, provides a tutorial overview of XQuery, and includes several examples of its use. 1 is considered the format of choice for the exchange of information among various applications on the Internet. The popularity of XML is due in large part to its flexibility for representing many kinds of information. The use of tags makes XML data self-describing , and the extensible nature of XML makes it possible to define new kinds of documents for specialized purposes. As the importance of XML has increased , a series of standards has grown up around it, many of which were defined by the World Wide Web Consortium (W3C). 2 For example, XML Sche-ma 3 provides a notation for defining new types of elements and documents; XML Path Language (XPath) 4 provides a notation for selecting elements within an XML document; and Extensible Stylesheet Language Transformations (XSLT) 5 provides a notation for transforming XML documents from one representation to another. XML makes it possible for applications to exchange data in a standard format that is independent of storage. For example, one application may use a native XML storage format, whereas another may store data in a relational database. Since XML is emerging as a standard for data exchange, it is natural that queries among applications should be expressed as queries against data in XML format. This use gives rise to a requirement for a query language designed expressly for XML data sources. In October 1999, W3C convened the XML Query Working Group 6 for the purpose of designing such a query language, to be called XQuery. XML data are different from relational data in several important respects that influence the design of a query language. Relational data tend to have a regular structure, which allows the descriptive meta-data for these data to be stored in a separate catalog. XML …