In this paper, we present an approach to knowledge discovery in databases in the context of object-based representation systems. The goal of this approach is to extract viewpoints and association rules from data represented by objects. A viewpoint is a hierarchy of classes (a kind of partial lattice) and an association rule can be defined within a viewpoint or between two classes lying in different viewpoints. The viewpoints construction algorithm allows to manipulate objects which are indifferently classes or instances of classes. Moreover, multivalued attributes as well as relations can be taken into account in the current approach.