Ban, Đoàn V., T. C. Tuấn, and Đoàn V. Thắng. “Approximate Answers to Null Queries With Incomplete Fuzzy Information in Object Oriented Database”. Journal of Computer Science and Cybernetics, vol. 27, no. 4, May 2012, pp. 329-40, doi:10.15625/1813-9663/27/4/597.