The XQuery Update Facility is a relatively small extension of the XQuery language which provides convenient means of modifying XML documents or data. As of March 14, 2008, the XQuery Update Facility specification has become a "Candidate Recommendation", which means it is now pretty stable.
Why an update facility in XML Query ? The answer seems obvious, yet after all the XQuery language itself - or its cousin XSLT2 - is powerful enough to write any transformation of an XML tree. Therefore a simple "store" or "put" function, applied to the result of such transformation, could seem sufficient to achieve any kind of database update operation. Well, perhaps. In practice this would be neither very natural, convenient, nor very efficient (such an approach requires storing back the whole document and makes optimizing very difficult). So as we will see the little complexity added by XQuery Update seems quite worth the effort.
The Instructions below give a quick yet comprehensive practical introduction to the XQuery Update extension, while highlighting some of its peculiarities.
Prerequisites: the reader is presumed to have some acquaintance with XML Query and its Data Model (the abstract representation of XML data, involving nodes of six types: document, element, attribute, text, comment, processing-instruction).