Cardinality of Relationship
One-to-One
Only one entity of the first set is linked to only one entity of the second set. E.g. A teacher teaches a student. Only one teacher is teaching only one learner.
One-to-Many
Only one entity of the first set is related to many entities of the second set. E.g. A teacher teaches learners.
Many-to-One
Multiple entities of the first set are related to many entities of the second set. E.g. Teachers teach a learner.
Many-to-Many
Multiple entities of the first set are related to multiple entities of the second set. E.g. Teachers teach learners