Mapping Cardinalities

Mapping Cardinalities, or cardinality ratios, express the number of entities to which another entity can be associated via a relationship set.

Mapping Cardinalities  are useful in describing binary relationships, although they can contribute to the description of relationship sets that involve more than two entity sets. In this section we shall concentrate on only binary relationship sets.

For a binary relationship set R between entities A and B, mapping cardinalities must be one of the following:

·         One-to-one: An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A.

·         One-to-many: An entity in A is associated with any number (zero or more) of entities in B. An entity in B, however, can be associated with at most one entity in A.

·         Many-to-one: An entity in A is associated with at most one entity in B. An entity in B, however, can be associated with any number (zero or more) of entities in A.

·         Many-to-many: An entity in A is associated with any number (zero or more) of entities in B, and an entity in B is associated with any number (zero or more) of entities in A.

           Fig 1: Mapping Cardinalities a) one to one b) one to many

         Fig 2: Mapping Cardinalities: a) many to one b) many to many