1. Salton, G., Buckley, C.: Improving Retrieval Performance by Relevance Feedback. Technical report, Ithaca, NY, USA (1988)
2. van Rijsbergen, C.J.: Information Retrieval. Butterworths, London (1979)
3. Codd, E.F.: A Database Sublanguage Founded on the Relational Calculus. In: SIGFIDET (ed.) ACM SIGFIDET Workshop on Data Description, Access and Control, pp. 35–61 (1971)
4. Date, C.J., Darwen, H.: A Guide to the SQL Standard, 3rd edn. Addison-Wesley, Reading (1993)
5. Zadeh, L.A.: Fuzzy Sets. Information and Control (8), 338–353 (1965)