Extending and Inferring Functional Dependencies in

kind of constraints in multidatabase systems. A major challenge is when component database schemas are schematic discrepant from each other, i.e., data values of one database correspond to schema labels of another. We propose “qualified functional dependencies” (or qualified FDs), an extension to conventional FDs to formalize integrity constraints in multidatabase systems. We first give inference rules to derive qualified FDs in fixed schemas, then study the derivation of qualified FDs during the transformations between schematic discrepant schemas. Propagation rules are given to derive qualified FDs of transformed schemas from qualified FDs of original schemas. Our work can be used in different stages of building and accessing a multidatabase system, e.g., to detect and resolve value inconsistency in schema integration, to verify lossless schema transformations, to normalize integrated schemas, to verify the integrity of data, and to optimize queries at an integration level. In particular, as an application of our theory, we will use FDs to check the validity of SchemaSQL views (SchemaSQL is a powerful multidatabase language).