3.3.1

What is an Entity What is an Entity Relationship Diagram? Different types of relationships

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.

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.

Strictly speaking... The is also an entity...as it too is made up of different attributes!

There are different types of relationship which you need to be aware of: – One-to-One – One-to-Many – Many-to-Many

Assuming a teacher only taught in one classroom in a school... Teacher Classroom

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

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

A third table is added to create two One-to- Many relationships. Teacher Students Class