TABLE OF CONTENTS The relational databases have been proved to be a very useful database model in information systems and have found wide applications successfully. However, the relational database model introduced by Codd (1970) does not deal with imprecise and uncertain data well. The data that is handled has to be either precise or NULL value. NULL value is used for all types of impreciseness such as unknown , not-applicable (many types of such impreciseness are cited in (ANSI/X3/SPARC, 1975). This model cannot model the real world accurately (Yazici and George, 1999).
Fuzzy data have been used to model imprecise information in databases since Zadeh introduced the concept of fuzzy sets (Zadeh, 1965), and the traditional relational databases have been thereby extended. Much of the work in the area has been in extending the basic model and query language to permit the representation and retrieval of imprecise data. A number of related issues such as data dependencies, implementation considerations and others have also been investigated in various fuzzy relational database models.
Three major extended relational models have been proposed as the fuzzy relational database models (Petry, 1996; Chen, 1999). The first one uses membership degrees of tuple belonging in [0, 1] instead of only in {0, 1} (Baldwin and Zhou, 1984; Raju and Majumdar, 1988). The second one uses the principle of replacing the ordinary equivalence among domain values by measures of nearness such as similarity relationships (Buckles and Petry, 1982) and proximity relationships (Shenoi and Melton, 1989). The third one directly...
