Entity Occurrence Diagrams

An entity-occurrence diagram illustrates the relationships between the entity occurrences of one entity, with the entity occurrences within a related entity. The creation of an entity occurrence diagram helps to identify the cardinality of the relationship that exists between the two entities.

In the diagram:

• Each entity is shown as a tall oval.

• Each entity occurrence is represented by the value of its identifier.

• Each relationship is illustrated by drawing a line between associated entity occurrences.

The following table indicates which School is managed by which Headteacher and which Headteacher manages which School. The entity-occurrence diagram can be drawn from this information.

From the diagram we can see that each occurrence in the School entity has an association with one occurrence in the Headteacher entity and each Headteacher has an association with only one occurrence in the School entity.

This confirms that there is a one-to-one relationship between the two entities.

The following table indicates which School employs which Teachers and which Teachers are employed by which School. The entity-occurrence diagram can be drawn from this information.

From the diagram we can see that each occurrence in the School entity has an association with one or more occurrence in the Teacher entity and each Teacher has an association with only one occurrence in the School entity.

This confirms that there is a one-to-many relationship between the two entities.

The following table indicates which Students study which Subjects and which Subjects are studied by which Students. The entity-occurrence diagram can be drawn from this information.

From the diagram we can see that each occurence in the Student entity has an association with one or more occurences in the Subject entity and each Subject has an association with one or more occurences in the Student entity.

This confirms that there is a many-to-many relationship between the two entities.

An entity occurrence diagram may be used to illustrate the relationship between multiple entities.