  In this work we establish and investigate connections between causality for
query answers in databases, database repairs wrt. denial constraints, and
consistency-based diagnosis. The first two are relatively new problems in
databases, and the third one is an established subject in knowledge
representation. We show how to obtain database repairs from causes and the
other way around. Causality problems are formulated as diagnosis problems, and
the diagnoses provide causes and their responsibilities. The vast body of
research on database repairs can be applied to the newer problem of determining
actual causes for query answers and their responsibilities. These connections,
which are interesting per se, allow us, after a transition -inspired by
consistency-based diagnosis- to computational problems on hitting sets and
vertex covers in hypergraphs, to obtain several new algorithmic and complexity
results for causality in databases.
