THUẦN, H.; HÀ, H. C. Relational algebra and a point of view for using null value in a fuzzy database model. Journal of Computer Science and Cybernetics, [S. l.], v. 17, n. 4, p. 1–10, 2012. DOI: 10.15625/1813-9663/17/4/2285. Disponível em: https://vjs.ac.vn/index.php/jcc/article/view/2285. Acesso em: 5 may. 2024.