简体繁体 English

有关BCNF分解的特定方案

[英]Specific scenario regarding BCNF decomposition

原文 2014-03-03 15:42:13 5 1 database/ database-design/ normalization/ database-normalization

Say I have a relation ABCD with FD's (A->D and AB -> ABCD) 假设我与FD有ABCD关系（A-> D和AB-> ABCD）

Will a decomposed relation ABC be in BCNF? 分解的关系ABC会出现在BCNF中吗？ According to the second FD, AB form a key and is therefore in BCNF, but if you only look at the FD A -> D, is the relation no longer in BCNF then? 根据第二个FD，AB构成一个密钥，因此位于BCNF中，但是如果仅查看FD A-> D，则该关系不再存在于BCNF中吗？

1 个解决方案

If you decompose a given relation schema (to which given dependencies apply), the next task is to determine, for each individual dependency in the original set : 如果分解给定的关系模式（给定的依赖项适用于该关系模式），则下一个任务是为原始集中的每个单独的依赖项确定：

(a) which (if any) of the new, decomposed, schemas does it apply to ? （a）它适用于哪些新的，已分解的模式？
(b) how has the decomposition affected the very definition of the FD ? （b）分解如何影响FD的定义？

Question (a) applies to your original A->D dependency. 问题（a）适用于您原始的A-> D依赖关系。 Question (b) applies, sort of, to your original AB->ABCD dependency. 问题（b）在某种程度上适用于您最初的AB-> ABCD依赖关系。 I say "sort of" because that version is quite "overstated". 我说“有点”是因为该版本相当“夸大其词”。 Given that A->D was already a given, it could just as well just say AB->C. 鉴于A-> D已经是给定的，也可以说AB-> C。