On the Expressive Power of XQuery-Based Update Languages

XQuery 1.0, the XML query language which is about to become a W3C Recommendation, lacks the ability to make persistent changes to instances of its data model. A number of proposals to extend XQuery with update facilities have been made lately, including a W3C Working Draft. In order to investigate some of the different constructs that are introduced in these proposals, we define an XQuery-based update language that combines them. By doing so, we show that it is possible to give a concise, complete and formal definition of such a language. We define subsets of this language to examine the relative expressive power of the different constructs, and we establish the relationships between these subsets in terms of queries and updates that can be expressed. Finally, we discuss the relationships between these subsets and existing XQuery-based update languages.