Summary

Data-base management systems are software components designed to store, retrieve, update, and control large amounts of permanent data. Up to now, they have mainly been used for business purposes to respond to a certain range of applications where data are precisely known (no imprecision, no uncertainty) and retrieval procedures are intended to obtain data according to their compliance with crisp conditions (whose satisfaction is not a matter of degree). The last few years have witnessed a tremendous increase in the use of computers in more and more domains, and the need for managing new kinds of data and for providing new capabilities for storage, access, and display. In this respect, one may imagine introducing what is often dubbed “uncertainty” into data bases. This term may refer to two main streams of problems. On the one hand, one wants to store and manipulate incomplete data (i.e., the available information about attribute values may be tainted with imprecision and/or uncertainty for some items). This can be achieved through different formalisms (probability [¹, ²] and possibility theory [³–⁵], in particular), but in that case, the retrieval process may also return results involving some uncertainty (if we are uncertain about the precise value of John's age, we cannot always be sure that John does (or does not) satisfy a given requirement in the context of a query selecting people on basis of their age). On the other hand, the term “uncertainty” is sometimes used to refer to flexible queries, because then one may assume that there is some ambiguity in their meaning. In fact, flexible queries are useful for describing preferences and thus for getting an ordered set of answers accordingly.