Entity Relationship Diagrams

3.3.1

This presentation covers

• What is an Entity

• What is an Entity Relationship Diagram?

• Different types of relationships

Introduction• Systems can be confusing things.

• To help people understand what they are trying to design they often use diagrams.

• There are different styles of diagrams including:– Entity Relationship Diagrams– State Transition Diagrams– Data Flow Diagrams– Flowcharts

• The type of diagram used will depend on the system being designed but a combination of diagrams can be used on the same project.

Entity Relationship Diagrams

• Also known as ERDs.

• This diagram represents entities and the relationships between them.

• We have already looked at these diagrams when we covered Normalisation back in AS Level...so this will be more of a refresher topic!

Entities

• Entities are ‘things’.

• Each entity is made up of attributes.

• For example, a is an entity.

• It is made up of leaves... and other things.

Entities

• Strictly speaking...

• The is also an entity...as it too is

made up of different attributes!

Types of Relationship

• There are different types of relationship which you need to be aware of:

– One-to-One– One-to-Many– Many-to-Many

One-to-One

• Assuming a teacher only taught in one classroom in a school...

Teacher Classroom

One-to-Many

• A teacher has many lessons but a lesson can only have one teacher.

Teacher Lesson

Many-to-Many

• A teacher has many students and a student can have many teachers.

• Many-to-Many relationships are never ideal so a third table is often created.

Teacher Students

Fixing Many-to-Many

• A third table is added to create two One-to-Many relationships.

Teacher Students

Class

Developing the ERD

• You must know the following things:

– The entities– The attributes for each entity– The primary key for the entity– The foreign key(s) in each entity

Something for you to do...

• A small guesthouse registers its guests on a database.

• Three of the entities used in the database are CUSTOMER, ROOM and BOOKING.

• Draw an ERD to represent this part of the database.