Hierarchies

This chapter, after a brief discussion regarding the abstraction process which created a hierarchy, gives an overview of the related works on the basic concepts of the different types of aggregation hierarchies, discussing the hierarchies from two different points of view: mapping between domain values and hierarchical structures, introducing the characterization of some OLAP operators on hierarchies and giving a set of operators that concern the change in the hierarchy structure. Finally, it proposes an enlargement of the operator set concerning hierarchies.