mapping cardinalities or constraints

Ø  Mapping Cardinalities (Constraints) - An E-R scheme may define certain constraints to which the contents of a database must conform.

1)     One- to- one

2)     One -to- many

3)     Many- to- one

4)     Many- to -many

1. 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. 

           
                   

2. One-to-many: An entity in A is associated with any number in B. An entity in B is associated with at most one entity in A. 

                   

3. Many-to-one: An entity in A is associated with at most one entity in B. An entity in B is associated with any number in A.

                     

4.       Many-to-many: Entities in A and B are associated with any number from each other.