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

Entity-relationship is a graphical representation of entities and their relationships to each other. Entity-relationship diagrams (ERDs) illustrate the logical structure of the database. Diagrams created to design these entities and relationships are called entity-relationship diagrams or ER diagrams.

Uses of entity-relationship diagram

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

Components of entity- relationship

Entity- Relationship diagrams have three different components;

1. Entities
2. Attributes
3. Relationships

1. Entities: Entities are objects or concepts within the data model. Each entity is represented by a box within the ERD. An entity might be considered a container that holds all of the instances of a particular thing in a system.
2. Attributes: a key attribute is the unique, distinguishing characteristics of the entity. The primary key is always the attribute of ER. 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.
3. Relationship: A relationship is an association among the instances of one or more entity types that are of interest to the organization. A relationship is an association between entities or entity occurrence

4. Cardinality: The cardinality defines the relationship between the entities in terms of numbers.

Types Of Cardinality Relationship

There are potentially three types of relationships which can exist between different entities;

1. One-to-One Relationship
2. One-to-Many Relationship
3. Many-to-Many Relationship

ASSIGNMENT 

1. What is Cardinality?
2. Mention and explain three types of relationships with examples.