What is a functional dependency?

• A. X -> Y means that Y is determined by X; values of X uniquely determine values of Y.
• B. Y -> X means that X is determined by Y.
• C. X and Y are unrelated.
• D. A functional dependency is a constraint on user access control.

Answer: (A)

In relational databases, a functional dependency expresses a relationship where the value of one set of attributes (X) uniquely determines the value of another set of attributes (Y). In X -> Y, if two rows have the same values for X, they must have the same values for Y. This means Y is a function of X, and knowing X lets you know Y with certainty. For example, if X is a student ID, Y could be the student name; the same ID should always yield the same name.

This is not about access control or security constraints, which is why a statement describing a functional dependency as a constraint on user permissions would be incorrect. It also isn’t about X and Y being unrelated; a functional dependency asserts a specific determinism—that identical X values force identical Y values. A reversed dependency (Y -> X) would be a different relationship, not the given one.