tailieunhanh - Database Management systems phần 6

Cho một tập của FDS và MVDs, nói chung, chúng ta có thể suy luận rằng FDS và MVDs thêm một số tổ chức. Một thiết lập âm thanh và đầy đủ các quy tắc suy luận bao gồm ba tiên đề Armstrong cộng với năm nguyên tắc bổ sung. Ba trong số các quy tắc bổ sung liên quan đến chỉ là MVDs | Schema Refinement and Normal Forms 447 Given a set of FDs and MVDs in general we can infer that several additional FDs and MVDs hold. A sound and complete set of inference rules consists of the three Armstrong Axioms plus five additional rules. Three of the additional rules involve only MVDs MVD Complementation If X Y then X R - XY. MVD Augmentation If X Y and W D Z then WX YZ. MVD Transitivity If X Y and Y Z then X Z - Y . As an example of the use of these rules since we have C T over CTB MVD complementation allows us to infer that C CTB CT as well that is C B. The remaining two rules relate FDs and MVDs Replication If X Y then X Y. Coalescence If X Y and there is a W such that W n Y is empty W Z and Y D Z then X Z. Observe that replication states that every FD is also an MVD. Fourth Normal Form Fourth normal form is a direct generalization of BCNF. Let R be a relation schema X and Y be nonempty subsets of the attributes of R and F be a set of dependencies that includes both FDs and MVDs. R is said to be in fourth normal form 4NF if for every MVD X Y that holds over R one of the following statements is true Y c X or XY R or X is a superkey. In reading this definition it is important to understand that the definition of a key has not changed the key must uniquely determine all attributes through FDs alone. X Y is a trivial MVD if Y c X c R or XY R such MVDs always hold. The relation CTB is not in 4NF because C T is a nontrivial MVD and C is not a key. We can eliminate the resulting redundancy by decomposing CTB into CT and CB each of these relations is then in 4NF. To use MVD information fully we must understand the theory of MVDs. However the following result due to Date and Fagin identifies conditions detected using only FD information under which we can safely ignore MVD information. That is using MVD information in addition to the FD information will not reveal any redundancy. Therefore if these conditions hold we do not even need to identify all MVDs. .