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This paper presents a relational database robust watermarking algorithm based on DCT transform, which takes the advantage of the frequency domain approach of multimedia digital watermarking into the area of relational database watermarking. By DCT transform on grouped and sorted tuples and modifying some of the transformed frequency coefficients to embed image watermark, only affecting one attribute for each tuple, the algorithm has good watermarking invisibility. The extraction does not need the original data. Experiments show that even if relational data suffers from approximately 40% of delete, add and modify attacks, we can still have a good robustness to extract the watermark.