Discriminated operations for interoperable databases

Federation Rules to be followed when associating external schemas into federated schemas are formulated. Discriminated operations in an object oriented model, such as discriminated generalization, and in the relational model, such as discriminated union and outer union, are introduced. It is then shown how they may help in obeying the Federation Rules. These operations allow multiple semantics in a single federated schema and thence minimize the number of federated schema needed, as well as their maintenance. They also allow easier solutions to other problems