Entity Relationship Diagrams (OCR A Level Computer Science)

Entity Relationship Types

What is an Entity?

• An entity in something worthy of capturing and storing data about e.g. students, orders, products, courses, customers

• Entities become tables in a relational database

• Relational databases store different entities in separate tables. Linking tables depends on the relationships between entities.

• There are 3 types of (sometimes called degrees of) relationships:

  • One to one

  • One to many

  • Many to many

• Imagine a company has 

  • A table of products

  • A table of customers

  • A table of the orders the customers have made

• What is the relationship between a customer and an order?

  • One customer can make multiple (many) orders

  • But each order relates to a specific (one) customer

  • So the relationship between customer and order is one to many

• Now consider the relationship between a product and an order

  • An order could have more than one (many) products on it

  • A product could be on more than one (many) order

  • So the relationship between order and product is many to many

• One to one relationships also exist but are not very common in databases

Drawing Entity Relationship Diagrams

• Entity Relationship Diagrams (ERDs) are simple diagrams that represent the entities (tables) that will be in a database and the relationships between these entities

• The entities are drawn as boxes with the entity name in

• The relationships are drawn in as what is known as ‘crow’s feet notation’

• This is how to draw the relationships in the exam:

• The names of the entities would go inside the boxes

Resolving Many to Many Relationships

• Many to many relationships cannot be implemented into an actual database and need to be ‘resolved’

• This involves creating a new table known as a ‘link’ table that goes between the entities

• This new table links the entities that have the many to many relationships together