In this chapter, you will learn:
½ The main characteristics of entity relationship components ½ How relationships between entities are defined, refined, and incorporated into the database design process ½ How ERD components affect database design and implementation ½ That real-world database design often requires the reconciliation of conflicting goals
This chapter expands coverage of the data-modeling aspect of database design. Data modeling is the first step in the database design journey, serving as a bridge between
real-world objects and the database model that is implemented in the computer. Therefore, the importance of data-modeling details, expressed graphically through entity relationship diagrams (ERDs), cannot be overstated. Most of the basic concepts and definitions used in the entity relationship model (ERM) were introduced in Chapter 2, Data Models. For example, the basic components of entities and relationships and their representation should now be familiar to you. This chapter goes much deeper and further, analyzing the graphic depiction of relationships among the entities and showing how those depictions help you summarize the wealth of data required to implement a successful design. Finally, the chapter illustrates how conflicting goals can be a challenge in database design, possibly requiring you to make design compromises.
F O U R
C H A P T E R
Because this book generally focuses on the relational model, you might be tempted to conclude that the ERM is exclusively a relational tool. Actually, conceptual models such as the ERM can be used to understand and design the data requirements of an organization. Therefore, the ERM is independent of the database type. Conceptual models are used in the conceptual design of databases, while relational models are used in the logical design of databases. However, because you are now familiar with the relational model from the previous chapter, the relational model is used extensively in this chapter to explain ER constructs and the way they are used to develop database designs.
4.1 THE ENTITY RELATIONSHIP MODEL (ERM)
You should remember from Chapter 2, Data Models, and Chapter 3, The Relational Database Model, that the ERM forms the basis of an ERD. The ERD represents the conceptual database as viewed by the end user. ERDs depict the database’s main components: entities, attributes, and relationships. Because an entity represents a real-world object, the words entity and object are often used interchangeably. Thus, the entities (objects) of the Tiny College database design developed in this chapter include students, classes, teachers, and classrooms. The order in which the ERD components are covered in the chapter is dictated by the way the modeling tools are used to develop ERDs that can form the basis for successful database design and implementation. In Chapter 2, you also learned about the various notations used with ERDs—the original Chen notation and the newer Crow’s Foot and UML notations. The first two notations are used at the beginning of this chapter to introduce some basic ER modeling concepts. Some conceptual database modeling concepts can be expressed only using the Chen notation. However, because the emphasis is on design and implementation of databases, the Crow’s Foot and UML class diagram notations are used for the final Tiny College ER diagram example. Because of its implementation emphasis, the Crow’s Foot notation can represent only what could be implemented. In other words: ¼ ¼ ¼
The Chen notation favors conceptual modeling. The Crow’s Foot notation favors a more implementation-oriented approach. The UML notation can be used for both conceptual and implementation modeling.
To learn how to create ER diagrams with the help of Microsoft Visio, see the Premium Website for this book: • Appendix A, Designing Databases with Visio...