Databases Illuminated, 4th Edition

6.3 Functional Dependency (FD)

A functional dependency (FD) is a type of relationship between attributes, as described in the following definition:

Definition: If R is a relation schema and A and B are non-empty sets of attributes in R, we say that B is functionally dependent on A if, and only if, each value of A in R has associated with it exactly one value of B in R.

We write this as

which we read as “A functionally determines B.” The definition says that if two tuples in an extension of R have the same value for A, they must also have the same values for B. More formally, for every pair of tuples, t₁ and t₂, in every instance of R, we have ...