XQuery is a W3C initiative, emerging standard, at the time of writing in draft stage. XQuery is targeted to be a universal query language for XML, without making any distinction where the data has come from. XQuery should bring the features that have existed in relational data access for years to XML data access. We have had tools like XPath and XSLT but their capabilities haven't been adequate for all needs and therefore some tasks have required the use of intermediate tools. This is what XQuery tries to cover and the initiative can be shortened by saying that XQuery should be the same for XML as SQL is for relational databases. However, by no means is XQuery's purpose to make existing standards obsolete; rather it provides a better tool to suit certain needs.
XQuery is interesting also from the .NET developer standpoint as the upcoming release of .NET Framework v2.0 (codename Whidbey) will also provide a better tool arsenal. One main new feature is the renewed data access architecture, especially from the XML standpoint. In Whidbey, the initiative to provide XML as a first class feature to software development has continued where it was left off in the previous version of the Framework. For example, in Whidbey there's support for middle-tier XML data access, which means one has the ability to query various data sources exposed through an XML provider as well as to manipulate the data.
The main new feature regarding this article is that Whidbey provides an XQuery implementation, giving us the capability to access heterogeneous data sources with the XML API and XQuery as a query language as it has been possible with a relational API and SQL. The main innovation in Whidbey is the common query processing architecture that is created for XSLT, XQuery, and XPath, giving them the same treatment (compilation, optimization, and MSIL) as is given to other CLR languages. If you want to know more about these, be sure to read A First Look at ADO.NET and System.XML v. 2.0 from Addison-Wesley, which I have reviewed in one of my previous articles. From this perspective, it was very interesting when I first read that Addison-Wesley publishes a book about XQuery. Now I have the book and here is the review for you.