Functional dependency
In relational database theory, a functional dependency (FD) is constraint between two attribute sets, whereby values in one set (the determinant set) determine the values of the other set (the dependent set). A functional dependency between a determinant set X and a dependent set Y can be described as follows: Given a relation R and attribute sets X,Y ⊆ {\displaystyle \subseteq } R, then X is said to functionally determine Y (written X → Y) if each X value is associated with precisely one Y value.