4.5. Unified data model and query languages

4.5.1. Introduction

The cornerstone of multiple data store integration solutions is some kind of unified data model. The latter is used to define what is commonly referred to as the “global” schema. This schema provides developers with transparent access to the different data stores. The definition of such a global schema has been widely addressed in the context of the relational data model, of the object-oriented data model [CAR 95] and of the semi-structured data models [PAP 95, MCH 97] (including XML-based data models). In all cases, the global schema is the result of an integration process which produces one single schema from several ones. This is a very complex process, especially when the data models are heterogeneous and when the number of data stores to integrate is high. To support the heterogeneity of the data models, a generic model is chosen that has enough expressivity to describe all of the varieties of the input...