(54)至(56)题基于以下叙述：
有关系模式P(A，B，C，D，E，F，G，H，I，J)，根据语义有如下函数依赖集：F =ABD→E，AB→G，B→F，C→J，C→I，G→H
现将关系模式P分解为三个关系模式P1(A，B，D，E，F)，P2(A，B，G，H)， P3(C，I，J)，则其中P2的规范化程度达到
A．1NF
B．2NF
C．3NF
D．BCNF
参考答案：B
解析： 将关系模式P分解为 3个关系模式P1(A，B，D，E，F)，P2(A，B， G，H)，P3(C，I，J)后，其中P2(A，B，G，H)的主码为(A，B) 。按照关系的规范化模式可知，关系P2能达到第一范式(1NF)，因为元组中的所有分量都是不可分的数据项；关系P2能达到第二范式(2NF)，因为不存在非主属性对码的部分函数依赖；但关系P2不能达到第三范式(3NF)， 因为非主属性G传递依赖于码。所以P2的规范化程度达到B(2NF)。
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| Have you ever regretted doing something you shouldn't have done or something you didn't do which you should have? At one time or another we probably all have. There's no 1 in getting depressed about it now-it's no 2 crying over spilt (溢出的) milk. However, there may be some gain in thinking about exactly what happened and why 3 we might be able to draw some conclusions for the future. One thing we all do now and again is to lose our 4 with a friend or close relative. The odd thing is that we more often display great 5 towards someone we are fond of than towards 6 . The explanation may be that we see friends and relatives as a kind of safety net, an opportunity to 7 a bit of steam in a safe environment, while the consequences (结果) of 8 a stranger could be far more serious. Being honest is usually thought of as a virtue (美德) and undoubtedly this is the 9 . On the other hand, we have all experienced occasions when we have spoken our minds to someone, telling them exactly what we feel, and then have found ourselves filled with feelings of 10 . Perhaps we should have kept our mouths shut? | ||||
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