Consistent Queries Over Databases with Integrity Constraints

INTRODUCTION Integrity constraints represent an important source of information about the real world. They are usually used to define constraints on data (functional dependencies, inclusion dependencies, etc.). Nowadays integrity constraints have a wide applicability in several contexts such as semantic query optimization, cooperative query answering, database integration and view update. Often databases may be inconsistent with respect to integrity constraints, that is, one or more integrity constraints are not satisfied. This may happen, for instance, when the database is obtained from the integration of different information sources. The integration of knowledge from multiple sources is an important aspect in several areas such as data warehousing, database integration, automated reasoning systems and active reactive databases. Since the satisfaction of integrity constraints cannot generally be guaranteed, in the evaluation of queries, we must compute answers which are consistent with the integrity constraints. Example 1 shows a case of inconsistency. Example 1: Consider the following database schema consisting of the single binary relation Teaches (Course, Professor) where the attribute Course is a key for the relation. Assume there are two different instances for the relations Teaches as reported in Figure 1.