Consistent Answers to SQL Queries

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As the amount of information available in online data sources explodes, there is a growing concern about the consistency and quality of answers to user queries. This project addresses the issue of using logical integrity constraints to gauge the consistency and quality of query answers. Although it is impractical to enforce global integrity constraints across different data sources and correct integrity violations by updating individual sources, integrity constraints capture important semantic properties of data. This project studies the formal notions of database repair and consistent query answer: a consistent answer is true in every minimal repair of the database. The information about answer consistency serves as an important indication of its quality and reliability. A variety of procedures for computing consistent query answers in the context of the relational data model and SQL are developed, and their computational complexity analyzed. The procedures exploit the properties of specific subsets of SQL and specific classes of integrity constraints. By providing information about query answer consistency, such procedures will enhance the functionality of existing DBMS in a non-intrusive way, particularly in the context of data integration applications.