Database outsourcing has become increasingly popular as a cost-effective solution to provide database services to clients. Previous work proposed different approaches to ensuring data integrity, one of the most important security concerns in database outsourcing. However, to the best of our knowledge, existing approaches require modification of DBMSs to facilitate data authentication, which greatly hampers their adoption in practice. In this paper, we present the design and implementation of an efficient and practical integrity assurance scheme without requiring any modification to the DBMS at the server side. We develop novel schemes to serialize Merkle B-tree based authentication structures into a relational database that allows efficient data retrieval for integrity verification. We design efficient algorithms to accelerate query processing with integrity protection. We further build a proof-of-concept prototype and conduct extensive experiments to evaluate the performance overhead of the proposed schemes. The experimental results show that our scheme imposes a low overhead for queries and a reasonable overhead for updates while ensuring integrity of an outsourced database without special support from server-side DBMSs.