What is a functional dependency in a relation?

A functional dependency is a property of the semantics of the attributes in a relation. The semantics indicate how attributes relate to one another and specify the functional dependencies between attributes. When a functional dependency is present, the dependency is specified as a constraint between the attributes. Consider a relation with attributes A and B, where attribute B is functionally dependent on attribute A. If we know the value of A and we examine the relation that holds this dependency, we will find only one value of B in all of the tuples that have a given value of A, at any moment in time. Note, however, that for a given value of B there may be several different values of A.

In the figure above, A is the determinant of B and B is the consequent of A.

The determinant of a functional dependency is the attribute or group of attributes on the left-hand side of the arrow in the functional dependency. The consequent of a functional dependency is the attribute or group of attributes on the right-hand side of the arrow.