In the last years, lots of work have been devoted to multidimensional modeling, star shape schemas and OLAP operations. However, "drill-across" has not captured as much attention as other operations. This operation allows to change the subject of analysis keeping the same analysis space we were using to analyze another subject. It is assumed that this can be done if both subjects share exactly the same analysis dimensions. In this paper, besides the implementation of an algebraic set of operations on a RDBMS, we are going to show when and how we can change the subject of analysis in the presence of semantic relationships, even if the analysis dimensions do not exactly coincide.

OLAP tools divide concepts based on whether they are used as analysis dimensions, or are the fact subject of analysis, which gives rise to star shape schemas. Operations are always provided to navigate inside such star schemas. However, the navigation among different stars is usually overlooked. This paper studies different kinds of Object-Oriented conceptual relationships (part of UML standard) between stars (namely Derivation, Generalization, Association, andFlow) that allow to drill across them.

This paper presents a multidimensional conceptual object-oriented model, its structures, integrity constraints and query operations. It has been developed as an extension of UML core metaclasses to facilitate its usage, as well as to avoid the introduction of completely new concepts. YAM/sup 2/ allows the representation of several semantically related star schemas, as well as summarizability and identification constraints.

"On-Line Analytical Processing" tools are used to extract information from the "Data Warehouse" in order to help in the decision making process. These tools are based on multidimensional concepts, i.e. facts and dimensions. In this paper we study the meaning of facts, and the dependencies in multidimensional data. This study is used to find relationships between cubes (in an Object-Oriented framework) and explain navigation operations.

According to the ANSI/SPARC architecture, external schemas are derived from the database conceptual schema. In object-oriented databases (OODBs), an external schema map include classes defined in the conceptual schema just as it may also contain derived classes which are directly or indirectly defined on the basis of conceptual schema classes. In the definition of derived classes in OODBs three main issues have to be resolved, namely: the integration of derived classes with other classes in an object schema; the definition of classes with new objects; and the transmission of modifications between the objects in derived classes and those of the classes from which they are defined. A solution to the first issue was proposed by Samos (1998). In this paper, the two remaining issues are further studied and new conceptual level solutions are put forward. Our proposals are informally illustrated through some examples.

The process of integrating the schemas of several databases into an integrated schema is not easy, due to semantic heterogeneities. We present a method/sup 1/ to detect class similarities by following a strategy and applying comparison criteria, that exploits the semantically rich structures of the schemas (previously enriched), along both the generalization/specialization and the aggregation dimensions. Relaxations may be applied to conform a pair of classes, resulting in penalizations in the computation of the degree of similarity. Our approach needs less comparisons than methods based on attribute comparison.>

An important aspect of any data model is the power to derive external schemata from the Database schema. Using and extended ERA metamodel in a systematic way, transformations from the Database schema into external schemata are classified in four classes, each classes having a number of subclasses. Determining for a specific data model which of these classes and subclasses of external schemata may be derived in that model, one gets a measure of its power in this respect. This method is applied to the basic relational model with views and nulls, and to the full basic Entity Relationship model with the "Burgundy algebra" of Parent & Spaccapietra.