What Is a Functional Dependency / Partial-Key Dependency / Transitive Dependency?

A functional dependency exists when one attribute, or set of attributes, uniquely determines another. Partial-key and transitive dependencies are specific dependency problems that normalization removes to reduce redundancy and update anomalies.

The Reasoning

A functional dependency is written as $X \rightarrow Y$. This means that whenever two records have the same value of $X$, they must also have the same value of $Y$. Here, $X$ is the determinant.

For example, in STUDENT(StudentID, StudentName), $\text{StudentID} \rightarrow \text{StudentName}$ because each student ID identifies one student name.

A partial-key dependency violates second normal form (2NF). It is removed by placing the partially dependent attribute in a separate relation, such as STUDENT(StudentID, StudentName).

By contrast, a full functional dependency uses the whole composite key. For example, in ENROLMENT(StudentID, CourseID, Grade), the grade depends on both identifiers, not either one alone.

A transitive dependency violates third normal form (3NF). It is removed by separating customer data into CUSTOMER(CustomerID, CustomerName) while retaining CustomerID as a foreign key in ORDER.

Normalization must also preserve valid links: the decomposed relations reconnect through primary-key and foreign-key attributes, avoiding accidental information loss. This is called lossless decomposition.

A common misconception is that every functional dependency is undesirable. Functional dependencies are normal and necessary; only dependencies that create partial or transitive reliance on a key cause normalization problems.

Exam Technique

For A3.2 Database design, identify the primary key, write dependencies using $X \rightarrow Y$, classify each dependency, and state the relevant normal form. Always explain how decomposition removes redundancy and prevents insertion, deletion, and update anomalies.