kc第八讲数据库规范化理论.ppt

Canonical Cover-要求 三大冗余现象：Sets of functional dependencies may have redundant dependencies that can be inferred from the others 依赖冗余现象：For example: A ? C is redundant in: {A ? B, B ? C} Parts of a functional dependency may be redundant 右方冗余现象：E.g.: on RHS: {A ? B, B ? C, A ? CD} can be simplified to  {A ? B, B ? C, A ? D} 左方冗余现象：E.g.: on LHS: {A ? B, B ? C, AC ? D} can be simplified to  {A ? B, B ? C, A ? D} 正则覆盖的直观要求：Intuitively, a canonical cover of F is a “minimal” set of functional dependencies equivalent to F, having no redundant dependencies or redundant parts of dependencies (冗余属性) (除不要求右方单属性外，等效于：[1]《数据库原理》王能斌，p.380,最小函数依赖集概念) 5.3 正则覆盖 5-9 什么是正则覆盖？ 返 回 Extraneous多余 Attributes Consider a set F of functional dependencies and the functional dependency ? ? ? in F. Attribute A is extraneous in ? if A ? ?  and F logically implies (F – {? ? ?}) ? {(? – A) ? ?}. Attribute A is extraneous in ? if A ? ?  and the set of functional dependencies  (F – {? ? ?}) ? {? ?(? – A)} logically implies F. Note: implication in the opposite direction is trivial微不足道的 in each of the cases above, since a “stronger” functional dependency always implies a weaker one. 即上述左、右方是相互蕴含的(等价的)! (学生作业) Example: Given F = {A ? C, AB ? C } B is extraneous in AB ? C because {A ? C, AB ? C} logically implies A ? C (I.e. the result of dropping B from AB ? C). Example: Given F = {A ? C, AB ? CD} C is extraneous in AB ? CD since AB ? C can be inferred even after deleting C 5.3 正则覆盖 5-10 正则覆盖中的冗余属性指什么？ Testing if an Attribute is Extraneous Consider a set F of functional dependencies and the functional dependency ? ? ? in F. 左方冗余属性的判定：To test if attribute A ? ? is extraneous in ? compute ({?} – A)+ using the dependencies in F check that ({?} – A)+ contains ?; if it does, A is extraneous in ? 右方冗余属性的判定： To test if attribute A ? ? is extraneous in ? compute ?+ using only the dependencies in