Entity-Relationship Models – SS2 Data Processing Lesson Note (First Term)

An entity-relationship (ER) model is a graphical representation of entities and their relationships to each other. It is drawn as an entity-relationship diagram (ERD), which illustrates the logical structure of a database. An ERD is made up of entities, attributes and relationships, and it is used to design the database.

Behavioural objectives

By the end of the lesson, students should be able to:

• define an entity-relationship diagram;
• state two uses of an entity-relationship diagram;
• name and describe the components of an entity-relationship diagram;
• explain cardinality;
• list the three types of cardinality relationship.

What are the uses of an entity-relationship diagram?

1. The ERD is used to identify the data that must be captured, stored and retrieved to support the business.
2. The ERD is also used to identify the data required to derive and generate a report on the performance measures that an organisation should be monitoring.

What are the components of an entity-relationship diagram?

Entity-relationship diagrams have three components:

1. Entities
2. Attributes
3. Relationships

Entities

Entities are objects or concepts within the data model. Each entity is represented by a box within the ERD. An entity can be considered a container that holds all of the instances of a particular thing in a system.

Attributes

Entities are further described by their attributes (sometimes called data elements). These are the smallest units of data that can be described in a meaningful manner. A key attribute is the unique, distinguishing characteristic of an entity. The primary key is always an attribute of the entity.

Relationships

A relationship is an association among the instances of one or more entity types that are of interest to the organisation. It is an association between entities or entity occurrences.

What is cardinality?

Cardinality defines the relationship between entities in terms of numbers.

Types of cardinality relationship

Three types of relationship can exist between different entities:

1. One-to-one relationship
2. One-to-many relationship
3. Many-to-many relationship

Evaluation

1. What is an entity-relationship diagram?
2. State two uses of an entity-relationship diagram.
3. Name the three components of an entity-relationship diagram.
4. What is an entity, and how is it shown in an ERD?
5. What is cardinality?

Assignment

1. What is cardinality?
2. Mention and explain the three types of cardinality relationship with examples.