In this paper, we propose a persistent watermarking technique of information systems supported by relational databases at the back-end. The persistency is achieved by identifying an invariant part of the database which remains unchanged w.r.t. the operations in the associated applications. To achieve this, we apply static data-flow analysis technique to the applications. The watermark is then embedded into the invariant part of the database, leading to a persistent watermark. We also watermark the associated applications in the information system by using opaque predicates which are obtained from the variant part of the database.