XQuery, 2nd Edition

Chapter 21. Working with Qualified Names, URIs, and IDs

This chapter describes the functions and constructors that act on namespace-qualified names, Uniform Resource Identifiers (URIs), and IDs. Each of these types has unique properties and complexities that sets it apart from simple strings.

Working with Qualified Names

The type xs:QName is used to represent qualified names in XQuery. An xs:QName value has three parts: a namespace, a local part, and an associated prefix. The namespace and the prefix are optional. If a QName does not have a namespace associated with it, it is considered to be in “no namespace.”

A prefix may be used to represent a namespace in a qualified name, for example, in an XML document. The prefix is bound to a namespace by using a namespace declaration. The prefix itself has no meaning; it is just a placeholder. Two QNames that have the same local part and namespace are equivalent, regardless of prefix. However, the XQuery processor does keep track of a QName’s prefix. This simplifies certain processes like serializing QNames and casting them to strings.

Most query writers who are working with qualified names are working with the names of elements and attributes. (It is also possible for a qualified name to appear as element content or as an attribute value, but this is less common.) You may want to retrieve all or part of a name if, for example, you want to test to see if it is a particular value, or you want to include the name in the query results. You may ...