a rela@on, X a set of a`ributes from R, A an a`ribute from R, F the set of FDs for R. • R is in 3NF if for all X → A in F+, – A ∈ X (trivial FD) or – X is a superkey or – A is part of some key • Nega3on: R is not in 3NF if there exists an X → A in F+, such that – A ∉ X (non-‐trivial FD) AND – X is not a key AND A is not part of some key • If R is in BCNF, obviously in 3NF. • If R is in 3NF, some redundancy is possible. It is a compromise, used when BCNF not achievable (e.g., no ``good’’ decomp, or performance considera@ons). Lipyeow Lim -‐-‐ University of Hawaii at Manoa 7