Key, participation and mapping constraints, Database Management System

Q. Explain the terms key constraints participation constraints and mapping caradinalities in ER model. 

Participation Constraints :   The participation of an entity set E in the relationship   set R is said to be total if every in E participates in at least one relationship in R. If only some entities in E participate in relationship in R, the participation of entity set E in relationship R is said to be partial. For eg., we expect every loan entity to be related to at least one customer through the borrower relationship. Therefore, the participation of loan in the relationship set borrower is total. In contrast, an individual can be a bank customer whether or not he/she has a loan with the bank. Hence, it is possible that only some of the customer entities are related to the loan entity set through the borrower relationship, and the participation of customer in the borrower relationship set is therefore partial.

Key Constraints: An important constraint on the entities of an entity type is key or uniqueness constraint on attributes. An entity type usually has an attribute whose values are distinct for each individual entity in the collection. Such an attribute is a key of an entity leans that the preceding uniqueness property must hold for every extension of entity type. Hence, it is a constraint that prohibits any two entities from having the same value for the key attribute at the same time. It is a constraint on all extensions of the entity type. Some entity types have more than one key attributes

Mapping constraints: Mapping constraints of mapping cardinalities or cardinality ratio, express the number of entities to which another entity can be associated via a relationship set. Mapping cardinalities are most useful in describing binary relationship sets that involve more than two entity sets. For a binary relationship set R between entities sets A and B the mapping cardinality must be one of the following: 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(see figure). One to Mant: An entity in A is associated with any number (Zero to more) of entities in B. An entity in B, however can be associated with at most one entity in A (see figure). Many to One: An entity in A is associated with at most one entity in B. An entity in B, however, can be associated with any number (zero of more) of entity in A (see figure) Many to Many : An entity in A is associated with any number of entities in B, and an entity cardinality for a particular relationship set obviously depends on the real word situation that the relationship set is modeling. As an illustration, consider the borrower relationship set. If in a particular bank a loan can belong to only one customer and customer can have several loans, then the relationship set from customer to loan is one to many. If a loan can belong to several customers the relationship set is many to many. 

Posted Date: 7/11/2012 4:10:48 AM | Location : United States







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