Relationship: category
The relationship category describes the relationship from the viewpoint of the target entity type. The category can be hierarchical, aggregating, referential or external.
Hierarchical:
The target entity type is identified by the source entity type and is also dependent on the source entity type for its existence. The key of the source entity type becomes part of the key of the target entity type. You may not change the relationship between two entities.
The relationship Offers with the cardinality 1:CN exists between the entity types Faculty (source entity type) and Courses (target entity type).
The source entity type Faculty has the attributes Faculty number (key attribute) and Faculty name.
The target entity type Courses has the attributes Faculty number (key attribute), Course number (key attribute), Number of the course instructor and Course title.
Since, with this category of relations hip, the source entity identifies the associated target entity by means of a part of the key attributes, a change to a relationship would mean that the target entity would be deleted and a new target entity would be created.
Aggregating:
The target entity type is identified by more than one source entity type and is dependent for its existence on the source entity type. The keys of the source entity types become part of the canonic key of the target entity type.
There continue to be special cases where the target entity type is not identified by one or more of the source entity types involved.
Referential:
The target entity type simply references the source entity type, that is, it is not identified by the source entity type. The key attributes of the source entity type are transferred to the target entity type as non-key attributes. The relationship between two entities may not be changed.
The relationship Teaches with cardinality 1:CN exists between the entity types Professor (source entity type) and Courses (target entity type).
The source entity type Professor has the attributes Number (key attribute), Name, Address, and Remuneration class.
The target entity type Courses has the attributes Faculty number (key attribute), Course number (key attribute), Number of the course instructor and Course name.
External:
A relationship between an entity type within a data model and an entity type outside the data model is described as an 'external' relationship.