The cardinality (n : m) describes the relationship with respect to the number of dependent entities (entities of the target entity type) and referenced entities (entities of the source entity type) in the relationship.
The possible values for the left and right sides of the cardinality have the following significance:
n = 1
For each dependent entity, there is precisely one referenced entity.
n = C
Dependent entities can exist that do not refer to an entity of the source entity type.
Referential relationships with the additional characteristic that a relationship can be defined or deleted for an entity irrespective of when the entity was created are referred to as temporarily referential.
m = 1
For each entity of the source entity type, there is exactly one dependent entity.
m = C
For each entity of the source entity type, there is a maximum of one dependent entity.
m = N
For each entity of the source entity type, there is at least one dependent entity.
m = CN
For each entity of the source entity type, there are any number of dependent entities.
Note that the Structured Entity Relationship Model (SERM) does not allow m:n relationships (that is, many-to-many relationships). Therefore, entity types in SERM can be arranged from left to right, from independent to dependent entity type.
The cardinality C:x (x = 1, C, CN, N) should be used only for relationships of the category 'referential'. (However, this cardinality is also possible for relationships of the category 'aggregating'.)
Hierarchical relationships do not allow this cardinality, since all dependent entities must refer to an entity of the source entity type.
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