Entity Relationship Model : Mapping Cardinalities

Mapping cardinalities :

Mapping cardinalities, or cardinality ratios, express the number of entities to which another entity can be associated via a relationship set. In other words, Cardinality defines the number of entities in one entity set, which can be associated with the number of entities of other set via relationship set. Mapping cardinalities are most useful in describing binary relationship sets, although they can contribute to the description of relationship sets that involve more than two entity sets. For a binary relationship set the mapping cardinality must be one of the following:

1. One-to-one :  One entity from entity set A can be associated with at most one entity of entity set B and vice versa. Consider, HOD of the Department. There is only one HOD in one department. That is there is 1:1 relationship between the entity HOD and Department.


2. One-to-many :  One entity from entity set A can be associated with more than one entities of entity set B however an entity from entity set B, can be associated with at most one entity.  For example one teacher teaches multiple classes.


 3. Many-to-one : More than one entities from entity set A can be associated with at most one entity of entity set B, however an entity from entity set B can be associated with more than one entity from entity set A.



4. Many-to-Many : This is a relationship where multiple instances of entities are related to multiple instances of another entity. A relationship between TEACHER and STUDENT is many to many. How? Multiple Teachers teach multiple numbers of Students. 




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