We present an approach to building libraries of "tasks" in complex action languages, such as Golog, for query answering. Our formalization is based on a situation calculus framework that allows probabilistic, temporal actions. Once a knowledge base is built containing domain knowledge including type information and a library of tasks and the goals they can achieve, we are interested in queries about the achievability of goals. We consider cases where, using domain object type and goal information in the KB, a user is able to get specific answers to a query while leaving some of the details for the system to figure out. In some cases where the specifics are missing from the KB, the user is provided with possible alternative answers that are compatible with the incomplete information in the KB. This approach is being explored in the context of a military operations planning domain for decision support.

• Thematic program: Data and Knowledge Bases (2006)
• WK student seminar

If we want to apply consistent query answering to a database implemented on a commercial DBMS, we need to consider the pecularities of those systems with respect to the way they handle null values. We revisit the problems of integrity constraint satisfaction and query answering in this setting. More specifically, we consider databases that contain the (only) null value NULL, and we propose a precise and uniform logical reconstruction of IC satisfaction and a semantics for it that is compatible with the way null values are treated according to the SQL standard. We also explore possible semantics for query answering in this kind of databases that are extensions of the one for IC satisfaction. Finally we present logic programs with stable model semantics that specifies the repairs obtained using this semantics for IC satisfaction and its corresponding notion of repair.

• Thematic program: Data and Knowledge Bases (2006)