Cloud Computing can be defined as a service or a platform, or an operating system over the Internet to perform tasks. Database has become a part and parcel of life and is being used in almost every computer application. As it is considered the most basic thing, Cloud computing offers this database service too. There are different cloud providers or platforms like Amazon, Google, Microsoft, and many more available in the market. Every cloud platform provides a database for the developers and each one of them has their own merits and demerits. In this paper the characteristics, architectures, advantages of Amazon's SimpleDB and Google's Big Table database are analyzed and discussed in detail. From the comparison of these databases, users can better understand the different cloud database and more reasonably choose what they want.