Functional Dependency

A functional dependency, written X → Y, means that the value of column (or columns) X determines the value of column Y: any two rows with the same X always have the same Y. For example, student_id → student_name.

Functional dependencies are the basis of normalization. A partial dependency (a column depending on only part of a composite key) violates Second Normal Form, and a transitive dependency (A → B and B → C, where B is not a key) violates Third Normal Form. Finding them shows how to split a table to remove redundancy.