The number of successful attacks on the Internet shows that it is very difficult to guarantee the security of online servers over extended periods of time. A breached server that is not detected in time may return incorrect query answers to users. In this article, we introduce authentication schemes for users to verify that their query answers from an online server are complete (i.e., no qualifying tuples are omitted) and authentic (i.e., all the result values are legitimate). We introduce a scheme that supports range selection, projection as well as primary key-foreign key join queries on relational databases. We also present authentication schemes for single- and multi-attribute range aggregate queries. The schemes complement access control mechanisms that rewrite queries dynamically, and are computationally secure. We have implemented the proposed schemes, and experiment results showed that they are practical and feasible schemes with low overheads.