Processing queries approximately is an effective way to reduce query
response time in on-line support systems, especially when the precise
answer is not necessary or early feedback is helpful. In an approximate
query processing system, a query is answered based on the data synopses
instead of the original data. The key of this research area is to
develop effective synopses construction techniques. In this monograph,
we will investigate various approximate query processing techniques
developed in the relational database, spatio-temporal database and in
the data stream environment.