Functional Dependencies (Cont.) Let R be a relation schema acR and BCR The functional dependency a→B holds on R if and only if for any legal relations r(R),whenever any two tuples f and t2 of r agree on the attributes a,they also agree on the attributes B.That is, t[o侧=t2[o→t[B]=t2[β] Example:Consider r(A,B with the following instance of r. 5 3 7 On this instance,A->B does NOT hold,but B->A does hold. Database System Concepts-6th Edition 8.12 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 8.12 ©Silberschatz, Korth and Sudarshan th Edition Functional Dependencies (Cont.) Let R be a relation schema   R and   R The functional dependency  →  holds on R if and only if for any legal relations r(R), whenever any two tuples t1 and t2 of r agree on the attributes , they also agree on the attributes . That is, t1 [] = t2 []  t1 [ ] = t2 [ ] Example: Consider r(A,B ) with the following instance of r. On this instance, A → B does NOT hold, but B → A does hold. 1 4 1 5 3 7