Lesson 5

In this lesson you will learn:

• To draw E-R diagrams consisting of entities, attributes and relationships using the E-R Diagrammer
• Understand the differences in modeling conventions/ notations of Hoffer, George & Valacich (1999) and Oracle Designer/2000

What is an E-R model?

An E-R model or and entity-relationship model is a logical representation of the data for a firm or business area. The E-R model consists of entities, relationships between or among entities, and the attributes of the entities. In order to create a detailed and fairly comprehensive E-R model, you will have to be clear about some of its relevant components, such as the entities, attributes, relationships, and the degree as well as the cardinality of a relationship. (You will see repeated references to these above mentioned terms in the course of this lesson).

Relevant terms in E-R modeling

An entity is a person, place, object or concept about which the organization wishes to maintain some data (e.g., STUDENT).

Attributes are certain properties of an entity that is of relevance to the firm (e.g. Student_ID).

Relationships are associations between the instances of one or more of the entities. e.g. Student TAKES Courses where Student and Courses are two entities and "TAKES" is the relationship between them.

Degree of a Relationship is the number of entity types that participate in a relationship. In the example given above, the degree is two (or binary), since there are two entity types that participate in the relationship. (Note that Designer/2000 allows you to create unary and binary relationships only).

Cardinality of a relationship (from entity A to entity B) is the number of instances of entity B that can be associated with each instance of entity A. (See Figure 5.1 for a graphical representation of the E-R model to be created).

Figure 5.1: A graphical representation of the E-R diagram to be created in this lesson

Creating the E-R Diagram using the E-R Diagrammer

1. To go to the E-R Diagrammer:
   • Select it from the Tools menu of the Repository Object Navigator (RON) (See Figure 5.2a) or select Entity Relationship from the Designer/2000 launchpad. (See Figure 5.2b).
     
     
     Figure 5.2a: Selecting the E-R Diagrammer from the Tools Menu of the RON
     
     
     Figure 5.2b: Selecting the E-R Diagrammer from the Designer/2000 launchpad

   • The new entity diagram window titled ERD1 will appear if you select the ERD from the Repository Object Navigator. (See Figure 5.3a).
     
     
     Figure 5.3a: The new entity diagram window ERD1 when you select the E-R Diagrammer from the RON

   • Otherwise, if you select the ERD from the Designer/2000 launchpad, the ERD will open with a blank screen. (See Figure 5.3b). In this case just go to File and select New. (See Figure 5.4). After you select New, you should see Figure 5.3a.
     
     
     Figure 5.3b: The E-R Diagrammer window when you select it from the Designer/2000 launchpad
     
     
     Figure 5.4: Selecting a new diagram from the E-R Diagrammer

   By this time, you should have a conceptual idea about the entities, the attributes and the relationships among the entities, that you would like to represent in the E-R diagram. For us in this project, our two main entities will be Student and Courses.

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