6.1. Introduction

The motivations for integrating user preferences into database queries are manifold [HAD 11]. First, it has become desirable to offer more expressive query languages that can be more faithful to what a user intends to say. Second, the introduction of preferences in queries provides a basis for rank-ordering the retrieved items, which is especially valuable in cases of large sets of items satisfying a query. Third, a classical query may also have an empty set of answers, while a relaxed (and thus less restrictive) version of the query might be matched by some items. A great deal of work has been carried out on preference queries in relational databases [STE 11] and more specifically concerning the fuzzy querying of relational databases, see, for instance, [PIV 12], which led in particular to a fuzzy extension of SQL, called SQLf [BOS 95]. However, even though relational databases are still widely used, the need to handle complex data has led to the emergence of other...