Data aggregation process plays a key function in wireless sensor networks. The overall aim of data aggregation techniques is to assemble and combine information in an energy efficient way so that the lifetime of the network can be increased. In order to increase the energy efficiency and network lifetime, many researchers have proposed in data aggregation in wireless sensor networks. This paper points to systematic study of subjects and challenges in data collection in wireless sensor networks. Data aggregation areas from six aspects: type of data aggregation techniques, secure data, privacy preserving data, cluster based data, energy efficient data and maximum lifetime of data aggregation.