Learning MySQL

The Entity Relationship Model

At a basic level, databases store information about distinct objects, or entities, and the associations, or relationships, between these entities. For example, a university database might store information about students, courses, and enrollment. A student and a course are entities, while an enrollment is a relationship between a student and a course. Similarly, an inventory and sales database might store information about products, customers, and sales. A product and a customer are entities, while a sale is a relationship between a customer and a product.

A popular approach to conceptual design uses the Entity Relationship (ER) model, which helps transform the requirements into a formal description of the entities and relationships that appear in the database. We’ll start by looking at how the Entity Relationship modeling process itself works, then apply it in Entity Relationship Modeling Examples” for three sample databases.

Representing Entities

To help visualize the design, the Entity Relationship Modeling approach involves drawing an Entity Relationship (ER) diagram. In the ER diagram, an entity set is represented by a rectangle containing the entity name. For our sales database example, the product and customer entity sets would be shown as in Figure 4-1.

Figure 4-1. An entity set is represented by a named rectangle

We typically use the database to ...