In relational database theory, a **functional dependency** is a constraint between two sets of attributes in a relation from a database.

Given a relation *R*, a set of attributes *X* in *R* is said to **functionally determine** another set of attributes *Y*, also in *R*, (written *X* → *Y*) if, and only 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, i.e. *Y* is a function of *X*. In simple words, if the values for the *X* attributes are known (say they are *x*), 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*.

The determination of functional dependencies is an important part of designing databases in the relational model, and in database normalization and denormalization. A simple application of functional dependencies is **Heath’s theorem**; it says that a relation *R* over an attribute set *U* and satisfying a functional dependency *X* → *Y* can be safely split in two relations having the lossless-join decomposition property, namely into where *Z* = *U* − *XY* are the rest of the attributes. (Unions of attribute sets are customarily denoted by mere juxtapositions in database theory.) An important notion in this context is a candidate key, defined as a minimal set of attributes that functionally determine all of the attributes in a relation. The functional dependencies, along with the attribute domains, are selected so as to generate constraints that would exclude as much data inappropriate to the user domain from the system as possible.

A notion of logical implication is defined for functional dependencies in the following way: a set of functional dependencies logically implies another set of dependencies, if any relation *R* satisfying all dependencies from also satisfies all dependencies from ; this is usually written . The notion of logical implication for functional dependencies admits a sound and complete finite axiomatization, known as **Armstrong's axioms**.

Read more about Functional Dependency: Properties and Axiomatization of Functional Dependencies, Irreducible Function Depending Set

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