A cloud database management system can be defined as a distributed database that allows us delivery of computing as service instead of product and sharing of resources as well as software between multiple devices over a network and has a tendency to grow significantly in future. In this paper we discuss about database management system as a cloud service. We focus on three important characteristics of cloud computing which are considered as the most troubled issues of cloud platform. We review cloud database challenges such as internet speed, multi-tenancy, privacy and security. At the end of this paper we also focused on the architecture of the database management system in a cloud.