A3.2.2 Constructing Entity Relationship Diagrams (ERDs) Notes

Constructing Entity Relationship Diagrams (ERDs)

The Significance of ERDs in Database Design

Entity Relationship Diagrams (ERDs) are visual representations of the structure of a database. They play a crucial role in:

1. Organizing Data: By clearly defining entities, attributes, and relationships.
2. Improving Communication: Providing a universal language for developers, designers, and stakeholders.
3. Preventing Errors: Allowing designers to identify and resolve issues before implementation.

Core Components of ERDs

1. Entities
   1. Definition: Objects or concepts that store data.
   2. Representation: Rectangles.
   3. Examples: Student, Course, Product.
2. Attributes
   1. Definition: Properties or characteristics of an entity.
   2. Representation: Ovals connected to their entity.
   3. Examples: A Student entity might have attributes like Name, ID, and Email.
3. Relationships
   1. Definition: Connections between entities.
   2. Representation: Diamonds or lines between entities.
   3. Examples: A Student "enrolls in" a Course.
4. Cardinality
   1. Definition: Describes the number of instances of one entity related to another.
   2. Types:
      1. One-to-One (1:1): Each instance of Entity A is related to one instance of Entity B.
      2. One-to-Many (1:N): Each instance of Entity A is related to multiple instances of Entity B.
      3. Many-to-Many (M:N): Multiple instances of Entity A are related to multiple instances of Entity B.
5. Modality (Optionality)
   1. Definition: Indicates whether a relationship is mandatory or optional.
   2. Examples:
      1. Mandatory: A Course must have at least one Student enrolled.
      2. Optional: A Student may or may not have a Scholarship.