问题 单项选择题

下列( )模式必定符合4NF。

A.关系R中只包含两个属性

B.关系R中只包含3个属性

C.关系R中都是主属性

D.关系R中都是主属性,且没有非平凡函数依赖

答案

参考答案:A

解析: 对于选项A).假设这个二目关系是R(A,B)。不失一般性,其函数依赖集可能有空集、F={A→B)、F={A→B,B→A)这3种情况.对于空集情况,此关系满足4NF;对于第二种情况,码是A,没有传递和部分函数依赖也没有违反4NF的条件,因此也是4NF。对于第三种情况,码是A或者B,没有传递和部分函数依赖,也满足每个函数依赖或者多值依赖关系的决定因素都包含码的条件,因此是4NF。综合以上3种情况,任何一个二目关系在函数依赖的范畴内至少能达到4NF。对于选项B),三目的关系不一定达到2NF。对于选项C)和D),一个全部是主属性的关系必然不会有非主属性部分或者传递依赖的问题,故至少是3NF。但选项C)无法保证对每一个非平凡函数依赖都包含码,选项D)无法保证对每一个非平凡的多值依赖都包含码。

阅读理解

Most of us are the only child in our family. We have no brothers or sisters. Our family mainly consists of our mom,our dad and us. We are growing up in a world differently from our parents’. And we will be the future leaders of China. Some day we have to work hand in hand with each other and make China more and more beautiful. So we must learn to love each other like brothers and sisters.

It’s hard to think how difficult life would be if we had a sibling(同胞,手足).We never have to share anything at home. Of course, we might have cousins, but we don’t have to live with them! Our parents try to give us the best, and the best doesn’t need sharing between two or three children. Do we ever ask ourselves if we are (宠坏) spoiled?

A recent study has found that many children don’t know how to love others! We don’r know the value of money. We don’t know how hard our parents are to support us. We don’t want others to regard us as spoiled, do we? Well, why not start now to help our parents at home, share a game with one of our friends or value the opinions of our classmates, even if we don’t always agree? In a word, we must learn to love each other!

小题1:The underlined phrase “consists of ” means ____________in Chinese.

A.由...组成

B.由...反映

C.由...提示

D.由...覆盖小题2:As Chinese teenagers, what’s our duty in the future?

A.To save money

B.To learn to love each other

C.To look after our brothers and sisters

D.To make China more and more beautiful小题3:Why do some people think we are spoiled?

A.We don’t know how to love each other

B.We only share games with our friends

C.We don’t know how to use money

D.Our parents work hard to support us.

单项选择题