Mapping Constraints in DBMS

The mapping constraint is the data constraint that expresses the number of entities to which another entity can be related via a relationship set.

These are most useful in describing the relationship sets that involve more than two entity sets.

For binary relationship set R on an entity set X and Y, there are four possible mapping cardinalities. These are as follows:

• One to one (1:1)
• One to many (1: M)
• Many to one (M:1)
• Many to many (M: M)

In one-to-one mapping (1:1):- That is an entity of entity-set X can be associated with at most one entity of entity-set Y and an entity in entity-set Y can be associated with at most one entity of entity-set X.

For Example - Whether Employee and Department are 2 entity sets, then we can define a one to one relationship ‘Manages’ between an Employee entity and a Department entity i.e. one employee manages at most one department and one department is managed by at most one employee.

One to many (1: M):- When An entity of entity-set X can be associated with at most one entity of entity-set Y and an entity in entity-set Y can be associated with any number of entities of entity-set X.

Many to one (M:1): When An entity of entity-set X can be associated with at most one entity of entity-set Y and an entity in entity-set Y can be associated with any number of entities of entity-set X.

Many to Many (M: M):- When An entity of entity-set X can be associated with any number of entities of entity-set Y and an entity in entity-set Y can be associated with any number of entities of entity-set X.

AND

These relations are used in DataBaseManagement System.