A functional dependency is a constraint between two sets of attributes from the database.
A functional dependency is a property of the semantics or meaning of the attributes.
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 fd is the attribute or group of attributes on the right-hand side of the arrow.