Querying and repairing inconsistent XML data

The problem of repairing XML data which are inconsistent and incomplete with respect to a set of integrity constraints and a DTD is addressed. The existence of repairs (i.e. minimal sets of update operations making data consistent) is investigated and shown to be undecidable in the general case. This problem is shown to be still undecidable when data are interpreted as "incomplete" (so that they could be repaired by performing insert operations only). However, it becomes decidable when particular classes of constraints are considered. The existence of repairs is proved to be decidable and, in particular, NP-complete, if inconsistent data are interpreted as "dirty" data (so that repairs are data-cleaning operations consisting in only deletions). The existence of general repairs (containing both insert and delete operations) for special classes of integrity constraints (functional dependencies) is also investigated. Finally, for all the cases where the existence of a repair is decidable, the complexity of providing consistent answers to a query (issued on inconsistent data) is characterized. © Springer-Verlag Berlin Heidelberg 2005.