Data Modeling Using the Entity-Relationship (ER) Model

PART 3

Conceptual Modeling and Database Design

Chapter 7

Data Modeling Using the Entity-Relationship (ER) Model

_Conceptual modeling is a very important phase in designing a successful database application. Generally, the term database application refers to a particular database and the associated programs that implement the database queries and updates. For exam-ple, a BANK database application that keeps track of customer accounts would include programs that implement database updates corresponding to customer deposits and withdrawals. These programs provide user-friendly graphical user interfaces (GUIs) utilizing forms and menus for the end users of the application— the bank tellers, in this example. Hence, a major part of the database application will require the design, implementation, and testing of these application programs. Traditionally, the design and testing of application programs has been considered to be part of software engineering rather than database design. In many software design tools, the database design methodologies and software engineering method-ologies are intertwined since these activities are strongly related.

In this chapter, we follow the traditional approach of concentrating on the database structures and constraints during conceptual database design. The design of application programs is typically covered in software engineering courses. We present the modeling concepts of the Entity-Relationship (ER) model, which is a popular high-level conceptual data model. This model and its variations are frequently used for the conceptual design of database applications, and many database design tools employ its concepts. We describe the basic data-structuring concepts and con-straints of the ER model and discuss their use in the design of conceptual schemas for database applications. We also present the diagrammatic notation associated with the ER model, known as ER diagrams.

Object modeling methodologies such as the Unified Modeling Language (UML) are becoming increasingly popular in both database and software design. These methodologies go beyond database design to specify detailed design of software modules and their interactions using various types of diagrams. An important part of these methodologies—namely, class diagrams—are similar in many ways to the ER diagrams. In class diagrams, operations on objects are specified, in addition to specifying the database schema structure. Operations can be used to specify the functional requirements during database design, as we will discuss in Section 7.1. We present some of the UML notation and concepts for class diagrams that are particularly relevant to database design in Section 7.8, and briefly compare these to ER notation and concepts. Additional UML notation and concepts are presented in Section 8.6 and in Chapter 10.

This chapter is organized as follows: Section 7.1 discusses the role of high-level conceptual data models in database design. We introduce the requirements for a sample database application in Section 7.2 to illustrate the use of concepts from the ER model. This sample database is also used throughout the book. In Section 7.3 we present the concepts of entities and attributes, and we gradually introduce the diagrammatic technique for displaying an ER schema. In Section 7.4 we introduce the concepts of binary relationships and their roles and structural constraints. Section 7.5 introduces weak entity types. Section 7.6 shows how a schema design is refined to include relationships. Section 7.7 reviews the notation for ER diagrams, summarizes the issues and common pitfalls that occur in schema design, and discusses how to choose the names for database schema constructs. Section 7.8 introduces some UML class diagram concepts, compares them to ER model concepts, and applies them to the same database example. Section 7.9 discusses more complex types of relationships. Section 7.10 summarizes the chapter.

The material in Sections 7.8 and 7.9 may be excluded from an introductory course. If a more thorough coverage of data modeling concepts and conceptual database design is desired, the reader should continue to Chapter 8, where we describe extensions to the ER model that lead to the Enhanced-ER (EER) model, which includes concepts such as specialization, generalization, inheritance, and union types (categories). We also introduce some additional UML concepts and notation in Chapter 8.