Abstract. Sensor database is a type of database management system which offers sensor data and stored data in its data model and query languages. In this system, when a user poses a query to this sensor database, the query will be disseminated across the database. During this process, each sensor generates data that match the query from its covered area and then returns the data to the original sensor. In order to achieve an energy-efficient implementation, it will be useful to select a minimally sufficient subset of sensors to keep active at any given time. Thus, how to find a subset efficiently is an important problem for sensor database system. We define this problem as sensor database coverage (SDC) problem. In this paper, we reduce the SDC problem to connected set cover problem, then present two approximation algorithms to select a minimum connected set cover for a given sensor database. Moreover, to guarantee robustness and accuracy, we require a fault-tolerant sensor database, w...