Typically, these databases are heterogeneous in that they store different types of data, represent data differently, use different software to manage the data and run on different computer hardware.
Effective decision making often requires access to data from multiple such databases.
Research on heterogeneous database management has focused on the development of mechanisms that can access data from multiple databases, while preserving the local autonomy of the databases, i.e., without making changes to the existing databases. Such representations are expected to hide the existence of multiple databases as well as the inherent heterogeneity in them from the user. Traditional approaches to heterogeneous database integration have attempted to resolve structural differences among the underlying databases. In the global schema approach, schemas corresponding to each local database are combined into a single integrated schema. In the federated approach, each local database provides an export schema, i.e., a portion of its schema that it is willing to share with other databases.