作者icant (脑残症候群发作....)
看板TransCSI
标题Re: [问题] 资料库系统
时间Tue Jan 22 11:07:12 2008
※ 引述《shadow216308 (无名shadow216308)》之铭言：
: 2.what undesirable dependencies are avoided when a relation is 2NF?
partial functional dependency
意即所有非键值完全功能相依於主键
: 3.consider a relation R(A.B.C.D.E) with the following dependencies;
: AB->C,CD->E,DE->B
: is AB a condidata key of this relation ? if not, is ABD? Explain your answer
^^^^^^^^^
candidate
AB不是；ABD是
候选键有唯一性(可唯一识别所有值组)和最小性(不可缩减)
只有AB的话无法决定E，ABD即可完全识别
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