Conditional XPath

摘要

XPath 1.0 is a variable free language designed to specify paths between nodes in XML documents. Such paths can alternatively be specified in first-order logic. The logical abstraction of XPath 1.0, usually called Navigational or Core XPath, is not powerful enough to express every first-order definable path. In this article, we show that there exists a natural expansion of Core XPath in which every first-order definable path in XML document trees is expressible. This expansion is called Conditional XPath. It contains additional axis relations of the form (child: :n[F])+, denoting the transitive closure of the path expressed by child: :n[F]. The difference with XPath's descendant: :n[F] is that the path (child: :n[F])+ is conditional on the fact that all nodes in between the start and end node of the path should also be labeled by n and should make the predicate F true. This result can be viewed as the XPath analogue of the expressive completeness of the relational algebra with respect to first-order logic.