DATABASE MANAGEMENT SYSTEM

MULTIVALUED AND JOIN DEPENDENCY IN DBMS

MULTIVALUED DEPENDENCY

A multivalued dependency exists when there are at least three attributes (like X,Y and Z) in a relation and for a value of X there is a well defined set of values of Y and a well defined set of values of Z. However, the set of values of Y is independent of set Z and vice versa.

Multivalued dependency would occur whenever two separate attributes in a given table happen to be independent of each other. And yet, both of these depend on another third attribute.

Because the lecturers attached to the course and the books attached to the course are independent of each other, this database design has a multivalued dependency; 

There are two multivalued dependencies in this relation: 

{course} ↠ {book} and equivalently {course} ↠ {lecturer}

JOIN DEPENDENCY

Join Dependency (JD) can be illustrated as when the relation R is equal to the join of the sub-relations R1, R2,…, and Rn are present in the database. Whenever we can recreate a table by simply joining various tables where each of these tables consists of a subset of the table’s attribute, then this table is known as a Join Dependency. 

 

We can decompose the table given above into these three tables given below. 

<Student_Skills>

<Student_Job>

 

<Job_Skills>

Our Join Dependency would be:

{(Name, Skills ), ( Name, Job), (Skills, Job)}

The relations given above have join dependency. It means that the join relation of the three relations given above is equal to the very original relation <Student>.