Which statement best describes a functional dependency X -> Y?

• A. X -> Y means that Y is determined by X; values of X uniquely determine values of Y.
• B. X -> Y means that X and Y are always equal.
• C. X -> Y means there is no relationship.
• D. X -> Y is a constraint on user permissions.

Answer: (A)

A functional dependency X -> Y expresses a determinism relationship: Y is determined by X. This means that if two rows in a relation share the same value for X, they must have the same value for Y. In other words, the values of X uniquely determine the value of Y.

For example, if X is a student ID and Y is the student’s email, the same student ID should always map to the same email. The key idea is that knowing X tells you exactly what Y must be.

X need not be the entire key, but if X is a key, then X -> all attributes holds, reinforcing that the value of X uniquely determines other attributes. The crucial point is the deterministic link from X to Y, not an assertion of equality between X and Y, no relationship at all, or any relation to permissions.