Tham khảo tài liệu 'database systems - part 11', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | COP 4710: Database Systems Spring 2004 Introduction to Normalization – Part 2 BÀI 11, 1 ngày School of Electrical Engineering and Computer Science University of Central Florida Instructor : Mark Llewellyn markl@ CC1 211, 823-2790 Phd, MS, Under Third Normal Form (3NF) is based on the concept of a transitive dependency. Given a relation scheme R with a set of functional dependencies F and subset X R and an attribute A R. A is said to be transitively dependent on X if there exists Y R with X Y, Y X X and Y A and A X Y. An alternative definition for a transitive dependency is: a functional dependency X Y in a relation scheme R is a transitive dependency if there is a set of attributes Z R where Z is not a subset of any key of R and yet both X Z and Z Y hold in F. Third Normal Form (3NF) A relation scheme R is in 3NF with respect to a set of functional dependencies F, if whenever X A holds either: (1) X is a . | COP 4710: Database Systems Spring 2004 Introduction to Normalization – Part 2 BÀI 11, 1 ngày School of Electrical Engineering and Computer Science University of Central Florida Instructor : Mark Llewellyn markl@ CC1 211, 823-2790 Phd, MS, Under Third Normal Form (3NF) is based on the concept of a transitive dependency. Given a relation scheme R with a set of functional dependencies F and subset X R and an attribute A R. A is said to be transitively dependent on X if there exists Y R with X Y, Y X X and Y A and A X Y. An alternative definition for a transitive dependency is: a functional dependency X Y in a relation scheme R is a transitive dependency if there is a set of attributes Z R where Z is not a subset of any key of R and yet both X Z and Z Y hold in F. Third Normal Form (3NF) A relation scheme R is in 3NF with respect to a set of functional dependencies F, if whenever X A holds either: (1) X is a superkey of R or (2) A is a prime attribute. Alternative definition: A relation scheme R is in 3NF with respect to a set of functional dependencies F if no non-prime attribute is transitively dependent on any key of R. Example: Let R = (A, B, C, D) K = {AB}, F = {AB CD, C D, D C} then R is not in 3NF since C D holds and C is not a superkey of R. Alternatively, R is not in 3NF since AB C and C D and thus D is a non-prime attribute which is transitively dependent on the key AB. Third Normal Form (3NF) (cont.) What does 3NF do for us? Consider the following database: assign(flight, day, pilot-id, pilot-name) K = {flight day} F = {pilot-id pilot-name, pilot-name pilot-id} Why Third Normal Form? flight day pilot-id pilot-name 112 317 Mark 112 Feb. 12 246 Kristi 114 317 Mark Why Third Normal Form? (cont.) flight day pilot-id pilot-name 112 317 Mark 112 Feb. 12 246 Kristi 114 317 Mark 112 Feb. 11 319 Mark Since {flight day} is key, clearly {flight .
