Redundancy, Dependencies and Normal Forms for XML Databases

With the advent of XML and its use as a database language, dependency and normal form theory has attracted novel research interest. Several approaches to build up a dependency and normal form theory for XML databases have been published, mainly concentrating on functional dependencies and keys. XMLlike database structures can be modelled by rational trees using constructors for lists and disjoint unions. This involves restructuring rules on subattributes. The absence of redundancy can be characterised by the nested list normal form. If ordering is ignored, constructors for sets or multisets have to be employed. For these the theory can be extended using counterfree functional dependencies. Finally, for keys an important research question is which systems of subattributes permit Armstrong instances. While this gives just a glimpse of a starting promising theory, a research agenda for further research will be set up.