Answers to Review Questions

1. Name and discuss the different levels of data abstraction as defined by ANSI/SPARC.

Conceptual

Located at the abstraction apex, the conceptual model represents a global view of the data. It forms the basis of the conceptual schema, which provides a relatively easily understood bird's-eye view of the data environment. Since the conceptual model focuses on the main data objects and avoids detail, it exhibits both software and hardware independence. The most widely used conceptual model is the Entity Relationship (E-R) model, which yields the basic database blueprint.

Internal

The internal model represents the adaptation of the conceptual model to a specific DBMS. Basically, the internal model requires the database designer to match the conceptual model's characteristics and constraints to those of the selected hierarchical, network, or relational DBMS. Therefore, although it is still hardware-independent, it is software-dependent.

External

The external model represents the applications programmer's view of the data environment. Its use requires that the modeler subdivide a universal set of requirements and constraints into functional modules, each represented by its own external model. (The modules correspond to business units such as production, sales, personnel, and so on.) Each external model is represented by its own external schema. Each business unit is thus represented by an external model that includes that unit's entities, the relationships between the entities, and its constraints. Since external models are defined for a specific DBMS, they are DBMS (software)-dependent, but hardware-independent.

The use of external schemas has several important advantages:

□ Using database subsets makes it easier to view the specific application program requirements.

□ The subsets make it easier to