In a world where the amount of electronic information available is constant
ly growing, techniques to select and filter information efficiently become
increasingly important. Continuous queries are a tool that allows users to
monitor one or more information sources, by giving the impression that the
queries are being run continually over them. In this paper, we formalize th
e notion of continuous queries for a wide spectrum of environments. We cons
ider both append-only data sources and systems that allow more general data
manipulation. We examine the case where the database management software m
ay be modified as well as where we must treat it as a black box. We study t
he classes of queries that can be supported in each case and present effici
ent implementation techniques far them.