Transitive dependency
In Database Management System, a transitive dependency is a functional dependency which holds by virtue of transitivity. A transitive dependency can occur only in a relation that has three or more attributes. Let A, B, and C designate three distinct attributes (or distinct collections of attributes) in the relation. Suppose all three of the following conditions hold:
- A → B
- It is not the case that B → A
- B → C
Then the functional dependency A → C (which follows from 1 and 3 by the axiom of transitivity) is a transitive dependency.
In database normalization, one of the important features of third normal form is that it excludes certain types of transitive dependencies. E.F. Codd, the inventor of the relational model, introduced the concepts of transitive dependence and third normal form in 1971.[1]
Example
A transitive dependency occurs in the following relation:
| Book | Genre | Author | Author Nationality |
|---|---|---|---|
| Twenty Thousand Leagues Under the Sea | Science Fiction | Jules Verne | French |
| Journey to the Center of the Earth | Science Fiction | Jules Verne | French |
| Leaves of Grass | Poetry | Walt Whitman | American |
| Anna Karenina | Literary Fiction | Leo Tolstoy | Russian |
| A Confession | Religious Autobiography | Leo Tolstoy | Russian |
The functional dependency {Book} → {Author Nationality} applies; that is, if we know the book, we know the author's nationality. Furthermore:
- {Book} → {Author}
- {Author} does not → {Book}
- {Author} → {Author Nationality}
Therefore {Book} → {Author Nationality} is a transitive dependency.
Transitive dependency occurred because a non-key attribute (Author) was determining another non-key attribute (Author Nationality).
Notes
- ↑ Codd, E.F. "Further Normalization of the Data Base Relational Model." (Presented at Courant Computer Science Symposia Series 6, "Data Base Systems," New York City, May 24th-25th, 1971.) IBM Research Report RJ909 (August 31st, 1971). Republished in Randall J. Rustin (ed.), Data Base Systems: Courant Computer Science Symposia Series 6. Prentice-Hall, 1972. See pages 45-51, which cover third normal form and transitive dependence.