Consistency of mappings Grigoris Karvounarakis University of Pennsylvania The vision of the SHARQ (Sharing Heterogeneous and Autonomous Resources and Queries) project involves the ability of peers to autonomously join the system and create new mappings between peer schemas. This raises the question whether there are cases in which adding a mapping could "break" the system, in the sense that there are no instances for the peers that satisfy all the mappings. We define the notion of consistency to capture this property of sets of mappings. We identify classes of mappings that are always consistent and provide an algorithm to decide consistency in other cases. In a related issue, we provide an algorithm to check if the result of evaluating a conjunctive query is always empty, over all instances that satisfy a consistent set of mappings; such queries need not be evaluated at all, regardless of the actual data in the peers, while identifying such queries allows the system to provide more accurate feedback to the user (instead of just returning an empty answer to the query).