Multivalued Dependency

MVD stands for Multivalued Dependency, which is a concept in database normalization theory. It describes a specific type of dependency between attributes in a relational database. Multivalued dependencies arise when an attribute or set of attributes determines a set of multiple values in another attribute, but not uniquely.

In other words, if two attributes, A and B, are multivalued dependent on each other, it means that for each value of A, there is a set of multiple corresponding values in B, and vice versa. However, the values in B are not necessarily unique for each value in A.

MVDs play a role in higher levels of normalization beyond the Third Normal Form (3NF). They are addressed when striving for Boyce-Codd Normal Form (BCNF) and Fifth Normal Form (5NF). The purpose of addressing MVDs is to further eliminate redundancy and anomalies in the database schema.

Here’s a simple example to illustrate a multivalued dependency:

Example: Student and Course Registration

Consider a database that tracks student course registrations. Each student can register for multiple courses, and each course can have multiple registered students. We’ll use a simplified version to illustrate the concept:

StudentID	CourseID
1	101
1	102
2	101

In this example, (StudentID, CourseID) is the primary key, and there’s a multivalued dependency between StudentID and CourseID.

Here’s what the MVD means in this context:

• For StudentID 1, there’s a set of courses {101, 102}.
• For StudentID 2, there’s a set of courses {101}.

Note that the set of courses is not unique for each student; StudentID 1 has the same set of courses as StudentID 2.

In higher normalization levels (BCNF and 5NF), you would address this multivalued dependency by decomposing the relation into separate relations, each representing a unique multivalued dependency.

MVDs are an advanced concept in database normalization and are typically addressed in complex database designs where achieving the highest levels of normalization and data integrity is crucial.