We address the problem of providing users with the ability to assess the integrity of join results produced by external
computational providers and computed over externally stored databases. Our approach relies on different mutually supporting
techniques offering strong integrity protection guarantees at a limited cost. The application of the approach is completely
transparent to the computational provider, against which data and query confidentiality is preserved. The paper introduces our
techniques analytically, examining their protection guarantees and performance. It also illustrates experimental results, which
confirm the effectiveness and efficiency of our solutions.