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XML Query, XQuery for short, is a new query language currently under development by the W3C. It is designed to query XML documents using a SQL-like syntax. XQuery’s capabilities go far beyond SQL however, because XML (and thus XQuery) isn’t bound to the rigid structure of tables and relations. XML can represent a large number of data models. Furthermore an XQuery query can return data from multiple documents in different locations. XSLT has similar capabilities, but many IT people will find XQuery much easier to understand, particularly database administrators familiar with SQL.

You can use XQuery to extract an XML document from a physical or virtual representation of XML data. An example of the latter is SQLXML (provided in Microsoft SQL Server 2000), which enables you to extract data from a SQL Server database formatted as XML using the HTTP protocol. Any system that exposes XML over HTTP is a potential source of data for XQuery. XQuery’s designers hope that XQuery can act as a unified query language for any data store, including XML files, XML databases, and non-XML data stores. With the proliferation of loosely coupled systems and data coming from half way across the globe, performance of multi-document queries is going to be an issue, particularly if you only need a small amount of data from a large document. Future versions of XQuery may alleviate this problem by distributing a query over the queried systems.

XQuery uses four main keywords to create query expressions: FOR, LET, WHERE, and RETURN. These keywords are commonly used in conjunction to query data and create a result. People familiar with XQuery who build an expression using these keywords refer to this as a FLWR-expression (or FLoWeR-expression). In technical terms, these expressions are element constructors – you use them to construct (sequences of) elements.

There are several applications providing the ability to query using XQuery. Microsoft has already hinted that the next release of SQL Server (codename Yukon) will provide support for XQuery as well, and both IBM and Oracle will likely offer some kind of XQuery support once XQuery attains W3C Recommendation status.

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You can read more about XQuery here.

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