Redundancy Free Mappings from Relations to XML

Given the fact that relational and object-relational databases are the most widely used technology for storing data and that XML is the standard format used in electronic data interchange, the process of converting relational data to XML documents is one that occurs frequently. The problem that we address in this paper is an important one related to this process. If we convert a relation to an XML document, under what circumstances is the XML document redundancy free? In some allied work we formally defined functional dependencies in XML (XFDs) and, based on this definition, formally defined redundancy in an XML document. We then introduced a normal form for an XML document (XNF) and showed that it is a necessary and sufficient condition for the elimination of redundancy. In this paper we address the problem of determining what class of mappings map a relation in BCNF to an XML document in XNF. The class of mappings we consider is very general and allows arbitrary nesting of the original flat relation. Our main result establishes a necessary and sufficient condition on the DTD induced by the mapping for it to be in XNF.