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Entity occurence

A specific example of an entity is called an instance or entity occurrence.

For example, Ola MacLeod, Hamish Robertson and Shan Ashiq are all entity occurrences found in the Student entity.

Maths, Computing and Physics are all entity occurrences within the Subject entity.

An entity-occurrence diagram shows the relationships between occurrences of a particular entity.

In an entity-occurrence diagram:

• each entity is shown as a tall oval
• inside each oval, an occurrence of an entity is shown by a dot and a description of the occurrence

Examples of entity-occurrence diagrams are shown below.

Example 1: One to one relationship

The following table and diagram show the headteachers that manage particular schools.

For this entity-occurrence diagram:

• each headteacher manages one, and only one school
• each school also has one and only one headteacher

This shows that there is a one-to-one relationship between these entities.

Example 2: One-to-many relationship

The following table and diagram show which teachers are employed at which schools:

For this entity-occurrence diagram:

• each school has multiple teachers employed
• each teacher only works at one school

This shows that there is a one-to-many relationship.

Example 3: Many-to-many relationship

The following table and diagram show which pupils study which subjects:

In this entity-occurrence diagram:

• each pupil can study multiple subjects
• each subject can have multiple pupils studying it

This shows that these entities share a many-to-many relationship.