Relationships between two entity types are defined by specifying the two entity types involved, that is, the source entity type (the existentially independent) and the target entity type (the existentially dependent entity type). The relationship is oriented; from the independent entity type to the dependent entity type. Relationships have these characteristics:

• category
• cardinality
• their business significance

A relationship category can be hierarchical, aggregating, referential or external.

There is a hierarchical relationship category between two entity types when the following apply:

• The target entity type is existentially dependent upon the source entity type, that is, the life span of a characteristic of the target entity type is shorter than or equal to the life span of the characteristic of the source entity type.
• The target entity type is generated from the source entity type, that is, the source entity type influences the characteristic values directly.
• The target entity type represents a semantic refinement, that is, the target entity type is a classification of the source entity type which describes the source entity type more precisely.

The key of the source entity type becomes part of the key of the target entity type. The relationship between two entities may not be changed.

There is an aggregating relationship category between two entity types when the following apply:

• The target entity type is existentially dependent on the source entity type, that is, the life span of a characteristic of the target entity type is shorter than or equal to the life span of the characteristic of the source entity type.
• The target entity type is generated from the source entity type, that is, the source entity type influences the characteristic values directly.
• At least one other source entity type, different from the first one, is involved in the formation of the target entity type.

The keys of the source entity types become part of the canonic key of the target entity type.

There is a referential relationship category when the following apply:

• The target entity type is existentially dependent on the source entity type.
• The source entity type determines the context of the target entity type, that is, an attribute group of the start entity type is present in the target entity type, but this does not generate the target entity type.

The key attributes of the source entity type are included in the target entity type as non-key-attributes. A relationship between two entities can be changed.

A distinction is made between strong and weak existential dependence. Strong existential dependence requires there to be an assignment to exactly one characteristic of the source entity type for each characteristic of the target entity type. If this condition applies only to one (time-dependent) subset of the target entity type, the existential dependence is said to be weak.

This results in the following situation:

A relationship is in the ‘external’ category if it is between an entity type within a data model and an entity type outside the data model.

The cardinality (n : m) describes the relationship with respect to the number of dependent entities (entities of the target entity type of the relationship) and referenced entities (entities of the source entity type) allowed for the relationship.

The possible values for the left and right sides of the cardinality have the following significance:

Each dependent entity has one referenced entity.

Dependent entities can exist that do not refer to an entity of the source entity type.

Each entity of the source entity type has one dependent entity.

Each entity of the source entity type has a maximum of one dependent entity.

Each entity of the source entity type has at least one dependent entity.

Each entity of the source entity type can have any number of dependent entities.