BCNF分解 [英] BCNF Decomposition

问题描述

我试图找出执行BCNF分解的正确步骤。我发现这个例子，但我不明白如何执行正确的步骤。

I am trying to figure out the correct steps in performing a BCNF decomposition. I found this example, but I do not understand how to perform the correct steps.

Schema =（A，B，C，D，E，F，G，H ）
FD's + {A - > CGH，AD-> C，DE-> F，G-> G}

Schema = (A,B,C,D,E,F,G,H) FD's + {A -> CGH, AD->C, DE->F, G->G}

有人可以显示正确的步骤吗？

Could someone show the correct steps?

推荐答案

使用您的FD确定最低覆盖率：

Determine a minimal cover using your FD's:

{A -> C, A -> G, A -> H, 
 B -> nothing, 
 C -> nothing,
 D -> nothing,
 E -> nothing,
 F -> nothing
 G -> nothing
 H -> nothing
 DE -> F}

注意 AD - > C 因为 A 独自决定 C ，意味着 D

Note AD -> C drops out because A alone determines C which implies D is redundant in the FD (see Armstrong's Axioms - Augmentation).

3NF和BCNF定义与关于密钥的依赖关系有关。这里唯一的复合键
是 DE 。 D 或 E 不参与任何其他非空FD的
，因此消除传递依赖，属性依赖于
'键，整个键，除了键之外什么都不是一个问题。

3NF and BCNF definitions relate to dependencies about compund keys. The only compound key you have here is DE. Neither D or E participate in any other non-null FD's so eliminating transitive dependencies and ensuring that dependent attributes rely on the 'key, the whole key, and nothing but the key' is not an issue here.

打破关系，使FD离开右侧是键，右侧是
是该键的非键依赖属性：

Break into relations so that the FD left hand side is the key and the right hand sides are the non-key dependent attributes of that key:

[Key(A), C, G, H]
[Key(D, E), F]

$ b b

现在从封面中删除这些属性，不管是独立的关系。

Now eliminate these attributes from the cover, whatever is left are standalone relations.

[Key(B)]

这应该在3NF / BCNF

This should be in 3NF/BCNF

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