It is with great pleasure that I present Michael Brundage's book about XQuery.

More than twenty years ago, several things happened in the still young field of computer science, which are all somehow related to the book you now keep in your hands: The relational model revolutionized the management of data and introduced the notion of declaratively querying data; the nested array theory influenced database researchers and APL language designers to develop declarative algebras to operate on hierarchical data such as nested relations; the notion of document markup was standardized as SGML and found wide deployment in the document management community; and computer language researchers developed the notion of functional languages and the theory of type systems.

Fast-forward to 1996: The database research community moved from object-model algebras that had their origin in the nested relational algebras to the field of semi-structure data management, which requires a data model that provides nesting of more complex and less regular structures. In the document management community, a couple of people, driven by the success of the SGML-based markup language HTML and the World-wide Web as well as by the complexity of SGML set out to produce an SGML-lite under the umbrella of the World-wide Web Consortium (W3C) that in 1997 came to be known as XML.

Luckily, the database researchers realized that XML provides the self-describing, hierarchical structure that is well-suited for managing semi-structure data, and some people in the document and database communities realized that the additional, more document-centric features of XML such as document order and mixed content actually seem to get us much closer to one of the holy grails of database and document management: a unifying model to describe structured, semi-structured data and document markup.

In 1998, the W3C organized a workshop to investigate whether there was a need for a query language on XML data. The response was overwhelmingly positive, and in late 1999 the XML Query working group was formed. Four years later, the working group is now finalizing the first version of the query language known as XQuery that combines important aspects of the last twenty-plus years of research in languages and data management such as declarativity, composability, functional semantics, and formal type systems.

When I first encountered XML in 1997, while working at Stanford University as a research associate on semi-structured data management and information integration, it quickly became clear to me that XML will play an important role in modeling markup and semi-structured data and serve as a lingua franca for information exchange. When the XQuery working group started, I joined the group as the primary representative for Microsoft Corporation and was lucky enough to be able to enlist Michael Brundage to build one of the first XQuery prototypes that helped us to understand and influence the design of the language.

This is not the only reason why Michael is highly qualified to write about XQuery. He is in the unique position of being an outsider with insider access and knowledge. This experience gives him a perspective on XQuery that should be very helpful for both the novice XQuery user, who wants to get to know the language, and the advanced XQuery user, who needs a comprehensive reference. He has written a book that is highly informative, logically organized, and goes beyond the pure reference characteristics of many programming and query language reference books. For instance, Michael identifies language properties and idioms that may surprise even advanced users of XQuery, and points out both the positive and negative aspects of a language which must satisfy a vast array of use cases, implementation environments, and interests.

With the publication of the W3C insider book XQuery from the Experts: A Guide to the W3C XML Query Language, Addison-Wesley is bringing you the ultimate collection of XQuery books that should be able to satisfy even the most inquisitive mind about XQuery.

Enjoy the book and keep it handy when you are in need of a dose of XQuery!

Michael Rys is a program manager in Microsoft's SQL Server team responsible for XML technologies. He is representing Microsoft in the ANSI SQL and SQL/XML standardization efforts and in the XQuery working group and publishes regularly on all aspects of XML. His interests include declarative, set-oriented languages, data and document management and information integration.