Thuần, Hồ, and Hồ Cẩm Hà. “Relational Algebra and a Point of View for Using Null Value in a Fuzzy Database Model”. <i>Journal of Computer Science and Cybernetics</i> 17, no. 4 (August 1, 2012): 1–10. Accessed January 12, 2026. https://jcc.vast.vn/jcc/article/view/2285.