Functional dependency

Relational database theory concept

In relational database theory, a functional dependency is the following constraint between two attribute sets in a relation: Given a relation R and attribute sets , X is said to functionally determine Y if each X value is associated with precisely one Y value. R is then said to satisfy the functional dependency X → Y. Equivalently, the projection is a function, that is, Y is a function of X. In simple words, if the values for the X attributes are known, then the values for the Y attributes corresponding to x can be determined by looking them up in any tuple of R containing x. Customarily X is called the determinant set and Y the dependent set. A functional dependency FD: X → Y is called trivial if Y is a subset of X.

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