Database of Scriptum

The E-R approach is not a relative, a derivative, or a generalization of the relational data model, it is not a data model at all but a design methodology, which can be applied (but is not limited) to the relational model.

Main components of the E-R approach are the concepts of entity and relationship. Entities model the objects that are involved in an enterprise—for example, the patients, physicians, and procedures in a hospital. Relationships model the connections among the entities—for example, professors teach courses. In addition,
integrity constraints on the entities and relationships form an important part of an E-R specification, much as they do in the relational model. For example, A physician/ a doctor can only perform a surgery at a given time on a given day.

An entity-relationship (E-R) diagram is a graphical representation of the entities, relationships, and constraints that make up a given design. It provides a graphical summary of the design that is extremely useful to the designer, not only in validating the correctness of the design but also in discussing it with colleagues and in explaining it to the programmers who will be using it. But unfortunately, there is no standard drawing convention for E-R diagrams, and hence there is a good deal of variation among database texts in many aspects of this approach.

After the enterprise has been modeled with E-R diagrams, there are ways of converting these diagrams into sets of CREATE TABLE statements. But unfortunately, this conversion process does not yield a unique schema, especially in the presence of constraints, because some constraints that can be indicated in the E-R diagrams have no direct counterparts in SQL.

An important advantage of this methodology is that the designer can focus on complete and accurate modeling of the enterprise, without worrying about efficiently executing the required queries and updates against the final database. Later, when the E-R diagrams are to be converted to CREATE TABLE statements, the designer can add efficiency considerations to the final table designs using normalization theory and other techniques.