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Constraints on ER Model in DBMS
Constraints are used for modeling limitations on the relations between entities.
There are two types of constraints on the Entity Relationship (ER) model −
Mapping cardinality or cardinality ratio.
Participation constraints.
Mapping Cardinality
It is expressed as the number of entities to which another entity can be associated via a relationship set.
For the binary relationship set there are entity set A and B then the mapping cardinality can be one of the following −
One-to-one
One-to-many
Many-to-one
Many-to-many
One-to-one relationship
An entity set 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 relationship
An entity set A is associated with any number of entities in B with a possibility of zero and an entity in B is associated with at most one entity in A.
Many-to-one relationship
An entity set A is associated with at most one entity in B and an entity set in B can be associated with any number of entities in A with a possibility of zero.
Many-to-many relationship
An entity set A is associated with any number of entities in B with a possibility of zero and an entity in B is associated with any number of entities in A with a possibility of zero.
Participation Constraints
Participate constraints are two types as mentioned below −
Total participation
Partial Participation
The participation constraints are explained in the diagram below −
Here, the customer to Loan is partial participation and the loan to the customer is total participation.
Total participation
The participation of an entity set E in a relationship set R is said to be total if every entity in E Participates in at least one relationship in R.
For Example − Participation of loan in the relationship borrower is total participation.
Partial Participation
If only some of the entities in E participate in relationship R, then the participation of E in R is said to be partial participation.
For example − Participation of customers in the relationship borrower is partial participation.
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