Multivalued Dependency exists in a relation when two attributes depend on the third attribute but independent to each other. It is a full constraint between two sets of attributes in a relation. It plays a vital role in 4NF. Multivalued Dependencies are also referred to as tuple generating dependencies.
A relation must have at least three attributes to have a multivalued dependency. It is because one attribute will determine at least the other two attributes. Suppose, for a dependency X ? Y, if for a single value of X, multiple values of Y exists, then the table may have a multivalued dependency.
Multivalued dependency is represented by the double arrow sign (??).
The relation student consists of three attributes student_id, Name, Course.
In the above relation, Name and Course are two independent
attributes in itself, but both are dependent on student_id.
In this case, these two attributes are multivalued dependent on student_id. Following are the representation of these dependencies:
student_id ?? Name
student_id ?? Course
The relation Employee consists of three attributes Name, Project, and Hobby.
In the above relation, Project and Hobby are two independent
attributes in itself, but both are dependent on Name attribute.
In this case, these two attributes can be called as multivalued dependent on Name. Following are the representation of these dependencies:
Name ?? Project
Name ?? Hobby