Example
Consider this example of a database of teaching courses, the books recommended for the course, and the lecturers who will be teaching the course:
| Course | Book | Lecturer |
|---|---|---|
| AHA | Silberschatz | John D |
| AHA | Nederpelt | William M |
| AHA | Silberschatz | William M |
| AHA | Nederpelt | John D |
| AHA | Silberschatz | Christian G |
| AHA | Nederpelt | Christian G |
| OSO | Silberschatz | John D |
| OSO | Silberschatz | William M |
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; if we were to add a new book to the AHA course, we would have to add one record for each of the lecturers on that course, and vice versa.
Put formally, there are two multivalued dependencies in this relation: {course} {book} and equivalently {course} {lecturer}.
Databases with multivalued dependencies thus exhibit redundancy. In database normalization, fourth normal form requires that either every multivalued dependency X Y is trivial or for every nontrivial multivalued dependency X Y, X is a superkey.
Read more about this topic: Multivalued Dependency
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