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Mapping Cardinality

A mapping cardinality is a data constraint that specifies how many entities an entity can be related to in a relationship set.

Example: A student can only work on two projects, the number of students that work on one project is not limited.

A binary relationship set is a relationship set on two entity sets. Mapping cardinalities on binary relationship sets are simplest.

Consider a binary relationship set R on entity sets A and B. There are four possible mapping cardinalities in this case:

  1. one-to-one - an entity in A is related to at most one entity in B, and an entity in B is related to at most one entity in A.

  2. one-to-many - an entity in A is related to any number of entities in B, but an entity in B is related to at most one entity in A.

  3. many-to-one - an entity in A is related to at most one entity in B, but an entity in B is related to any number of entities in A.

  4. many-to-many - an entity in A is related to any number of entities in B, but an entity in B is related to any number of entities in A.

Resolve Many-To-Many Relationships

Many-to-many relationships cannot be used in the data model because they cannot be represented by the relational model. Therefore, many-to-many relationships must be resolved early in the modeling process. The strategy for resolving many-to-many relationship is to replace the relationship with an association entity and then relate the two original entities to the association entity.

Example:Employees may be assigned to many projects. Each project must have assigned to it more than one employee.

A many to many recursive relationship is resolved in similar fashion.