Two most important type of mapping constraints are Mapping cardinalities and Existence Dependencies. For a binary relationship set R between entity sets A and B, The mapping cardinalities must be one of the following:
o One to One: An entity in A is associated to at most one entity in B and vice-versa.
o One to Many: An entity in A is associated with any number of entities in B but an entity in B is associated to at most one entity in A.
o Many to One: An entity in A is associated to only one entity in B while an entity in B is associated to any number of entities in A.
o Many to Many: An entity in A is related to any number of entities in B and viceversa.
If the existence of entity x depends on the existence of entity y, then x is said to be existence dependent on y. If y is deleted so is x. y is dominant entity and x is subordinate entity. Example a payment entity is dependent on the loan entity. Payment are identifies by payment number, payment date, and payment amount. Weak entities are existent dependent on their identifying owner. But every dependent entity may not be a weak entity. In the above example of loan and payment entities payment entity is having the payment number as unique key.
