On the power to derive external schemata from the database schema

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.