We consider the following scenario for a mapping system: given a source schema, a target schema, and a set of value correspondences between these two schemas, generate an ex-ecutable transformation (i.e., a set of queries) to compute target instances from source instances. We base this compu-tation on two main components: (i) a schema mapping gen-eration algorithm, to compute a declarative schema mapping from the correspondences, and (ii) a query generation algo-rithm, to compute a transformation from the schema map-ping. In this paper, we introduce novel schema mapping and query generation algorithms for mappings between relational schemas with keys, foreign keys and nullable attributes. We extend current relational mapping algorithms (e.g., those proposed in the Clio framework), which are able to deal only in a more limited way with such integrity constraints. As a further contribution, we propose referenced-attribute corre-spondences, which permit to specify more precise mappings than traditional attribute correspondences, while retaining a simple and intuitive semantics. 1.

DaWaII (Data Warehouse IntegratIon) is a tool supporting the various activities related to the integration of multidimensional data. This problem arises in common scenarios where there is the need to combine independently developed data warehouses. Actually, a today common practice for building a data warehouse is to develop a collection of integrated data marts, each of which provide a dimensional view of a single business process. These data marts should be based on shared dimensions but very often, even within the same company, designers develop their data marts independently and it turns out that their integration is a difficult task. Indeed, the problem arises in other common cases. For instance, when different companies get involved in a federated project or when there is the need to combine a proprietary data warehouse with external information, for instance, with multidimensional data wrapped from the Web.

We report on the design of a novel architecture for data warehousing based on the introduction of an explicit "logical" layer to the traditional data warehousing framework. This layer serves to guarantee a complete independence of OLAP applications from the physical storage structure of the data warehouse and thus allows users and applications to manipulate multidimensional data ignoring implementation details. For example, it makes possible the modification of the data warehouse organization (e.g. MOLAP or ROLAP implementation, star scheme or snowflake scheme structure) without influencing the high level description of multidimensional data and programs that use the data. Also, it supports the integration of multidimensional data stored in heterogeneous OLAP servers. We propose [Formula: see text], a simple data model for multidimensional databases, as the reference for the logical layer. [Formula: see text] provides an abstract formalism to describe the basic concepts that can be found in any OLAP system (fact, dimension, level of aggregation, and measure). We show that [Formula: see text] databases can be implemented in both relational and multidimensional storage systems. We also show that [Formula: see text] can be profitably used in OLAP applications as front-end. We finally describe the design of a practical system that supports the above logical architecture; this system is used to show in practice how the architecture we propose can hide implementation details and provides a support for interoperability between different and possibly heterogeneous data warehouse applications.

In the context of object databases, we study the application of an update method to a collection of receivers rather than to a single one. The obvious strategy of applying the update to the receivers one after the other, in some arbitrary order, brings up the problem of order independence. On a very general level, we investigate how update behavior can be analyzed in terms of certain schema annotations, called colorings. We are able to characterize those colorings that always describe order-independedent updates. We also consider a more specific model of update methods implemented in the relational algebra. Order-independence of such algebraic methods is undecidable in general, but decidable if the expressions used are positive. Finally, we consider an alternative parallel strategy for set-oriented applications of algebraic update methods and compare and relate it to the sequential strategy.

We propose new criteria for the classification of systems for database cooperation, based on the nature of the component databases. In fact, the traditional criteria - heterogeneity, distribution, and autonomy - are often constraints for the design process, rather than design parameters. In this case, other features of the component databases should be addressed. We claim that a more useful classification can be based on three new criteria: (a) degree of transparency, (b) complexity of operations, and (c) level of liveliness of data. This leads us to distinguish three main categories of systems: (i) multidatabases, (ii) data warehouses, and (iii) local information systems with external data. For each of these categories, we discuss implementations based on tools offered by currently available technology

We address the issue of designing effective query languages for OLAP databases. The basis of our investigation is MD, a new data model for multidimensional databases that, unlike other multidimensional models, is independent of any specific implementation and as such provides a clear separation between practical and conceptual aspects. In this framework, we present and compare two query languages, based on different paradigms, for OLAP databases. The first language is algebraic and provides an effective way to manipulate multidimensional data in a procedural fashion. Although this language is clean and powerful, it is clearly not suited for final users. We therefore propose a high-level graphical language that allows the user to specify analytical queries in a natural and intuitive way. It turns out that the two languages have the same expressive power.

In the context of object databases, we study the application of an update method to a collection of receivers rather than to a single one.The obvious strategy of applying the update to the receivers one after the other, in some arbkrary order, brings up the problem of order independence.On a very general level, we investigate how update behavior

The IsaLog model and language are presented. The model has complex objects with classes, relations, and is a hierarchies. The language is strongly types and declarative. The main issue is the definition of the semantics of the language, given in three different ways that are shown to be equivalent: a model-theoretic semantics, a reduction to logic programming with function symbols, and a fixpoint semantics. Each of the semantics presents new aspects with respect to existing proposals because of the interaction of oid-invention with general is a hierarchies. The solutions are based on the explicit Skolem functors, which provide a powerful tool for manipulating object-identifiers.>