One to One(1: 1)
A one-to-one relationship exists when only one instance of an individual is connected with at most one entity of another (same) entity set via relationship instance.
An entity(tuple) in E1 is associated with at most one entity(tuple) in E2, and an entity in E2 is associated with at most one entity in A.
Example:
Candidate keys of relation R(A, B) = A, B
One to Many (1: M)
A one-to-many relationship exists when only one instance of an entity is connected with many entities of another entity set via relationship instances.
An entity(tuple) in E1 is associated with zero or more entities in E2, but an entity in E2 can be associated with at most one entity in E1.
Example:
Candidate key of R (A, B) = B
Many to One (M: 1)
A many-to-one relationship exists when more than one instance of the entity set is on the left and only one instance of the entity set on the right is associated with the relationship.
An entity(tuple) in Ę2 is associated with zero or more entities in E1 but an entity in
E1 can be associated with at most one entity in E2.
Example:
Candidate key of R (A, B) = A
Many to Many (M: M)
A many-to-many relationship exists when more than one instance of the entity set is on the left and more than one entity of another entity set is on the right is associated with the relationship.
An entity(tuple) in E2 is associated with zero or more entities in E1, and an entity(tuple) in E1 is associated with zero or more entities in E2.
Example:
Candidate key of R(A, B) = AB
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