  Wireless sensor networks become integral part of our life. These networks can
be used for monitoring the data in various domain due to their flexibility and
functionality. Query processing and optimization in the WSN is a very
challenging task because of their energy and memory constraint. In this paper,
first our focus is to review the different approaches that have significant
impacts on the development of query processing techniques for WSN. Finally, we
aim to illustrate the existing approach in popular query processing engines
with future research challenges in query optimization.
