DATABASE DESIGN



Database Design



Database Design

Introduction

The ability of a database designer to understand and model the information that an organization uses is a critical design skill. Data modelers use a variety of tools and techniques to understand an organization's data. In order to understand how to properly model data, you must become familiar with a modeling approach known as entity relationship modeling, which is the subject to of this chapter. (Piessens , 2005, pp. 156-173)

Entity-relationship (E-R) modeling is one approach to semantic modeling. When database designers attempt to understand and represent meaning, they are engaged in semantic modeling, which can help in making database design more systematic (Date, 1990). Although a number of approaches to semantic modeling exist, this chapter focuses on entity relationship modeling. More specifically, we use E-R modeling to carry out conceptual data modeling

ER modeling, introduced by Chen (1976) consists of a number of activities that help database designers understand the objects the organization wants to store information about, the important characteristics of these objects and the associations among various objects.

The Entity Relationship Diagram

The end result of E-R modeling is the E-R diagram (ERD), a graphical representation of the logical structure of a database. An ERD serves several purposes. First, the database analyst/designer gains a better understanding of the information to be contained in the database through the process of constructing the ERD. Second, the ERD serves as a documentation tool. (Barker , 2000, pp. 247) Finally, the ERD is used to communicate the logical structure of the database to users. useruser In particular, the ERD effectively communicates the logic of the database to users.

Entities

The E-R modeling process identifies three basic elements: entities, attributes and relationships. (Peter , 2006, pp. 9-36 )

An entity is a thing that can be distinctly identified (Chen, 1976). In database design, entities are the “things” about which the database stores information. Entities can include, but are not limited to

·         tangible items, such as equipment,

·         concepts, such as accounts,

·         people, such as employees

·         events, such as sales, or

·         places, such as business locations.

The term entity type refers to a number of related items, while an entity instance refers to a single occurrence of an entity type. For example, employee number 123-45-6789 refers to a single occurrence, or entity instance, of the entity EMPLOYEE. The term entity refers to entity type. In addition, note that entity occurrence is sometimes used rather than entity instance. In ER diagrams, rectangles represent entities, as shown in Figure 1.

Figure 1: Entity Examples

Attributes

An attribute is a single data value that describes a characteristic of an entity. Other terms such as data item and field, describe the same essential concept.

Each entity has a corresponding set of attributes that represent the information about the entity that the organization is interested in. For example, a university may wish to know the name, address, phone number, and major of each student. Put in database terms, STUDENT is the entity of interest, and NAME, ADDRESS, PHONE, and MAJOR are the attributes of ...