- Constraint between two sets of attributes
- Formal method for grouping attributes
- DB as one single universal relation/-literal
- R = {A
_{1},A_{2},…,A_{n}} - Two sets of attributes, X subset R,Y subset R
- Functional dependency (FD or f.d.) X -> Y
- If t
_{1}[X] = t_{2}[X], then t_{1}[Y] = t_{2}[Y] - Values of the Y attribute depend on value of X
- X functionally determines Y, not reverse necessarily