FD Covers and Universal Complements of Simple Projections

Abstract. The constant-complement strategy, in which the admissible updates to a given view are those which hold a second complementary view constant, remains one of the most attractive formalisms for identifying suitable translation mechanisms for updates to views of database schemata. However, in general, it suffers from the drawback that the reflections of view updates to the main schema can depend upon the choice of complement in various ways. To overcome this drawback completely, a special kindofcomplement, calledauniversalcomplement,is required.In this paper, sufficient conditions for the existence of such a complement are established for a classical but nevertheless very important setting — views defined by simple projection of a universal relational schema constrained by functional dependencies (FDs). Certain uniqueness properties of covers of these dependencies prove critical in the characterization. The results are extended to quasi-universal complements, which are unique up to exchange of equivalent attributes, thus recapturing certain situations for which unique covers do not exist.