Concept:
A functional dependency is a constraint between two sets of attributes from the database.
Explanation:
In general form, a functional dependency denoted by X -> Y between two sets of attributes X and Y that are subsets of R specifies a constraint on the possible tuples that can form a relation state r of R.
The constraint is that for any two tuples T1 and T2 in r that have T1[X] = T2[Y], they must also have T1[Y]
= T2[Y]. It means that values of Y component of a tuple in r depend on or are determined by the value of X component.
If a relation R contains attributes (X and Y). Then X -> Y is a functional dependency. It means X functionally determines Y in R if and only if two tuples agree on their X value, they must necessarily agree on their Y value.
A functional dependency is a property of semantics or meaning of the attributes.

