Temporal Query Languages (Cont.) Functional dependencies must be used with care:adding a time field may invalidate functional dependency A temporal functional dependency x->holds on a relation schema R if,for all legal instances r of R,all snapshots of r satisfy the functional dependency X->Y. SQL:1999 Part 7(SQL/Temporal)is a proposed extension to SQL:1999 to improve support of temporal data. 6 Database System Concepts,5th Ed. 24.6 ©Silberschat乜，Korth and SudarshanDatabase System Concepts, 5 24.6 ©Silberschatz, Korth and Sudarshan th Ed. 6 Temporal Query Languages (Cont.) Functional dependencies must be used with care: adding a time field may invalidate functional dependency A temporal functional dependency x → Y holds on a relation schema R if, for all legal instances r of R, all snapshots of r satisfy the functional dependency X →Y. SQL:1999 Part 7 (SQL/Temporal) is a proposed extension to SQL:1999 to improve support of temporal data. 