问题 单项选择题

若关系模式中存在非主属性对码的部分依赖,则该关系模式属于 (11)

A.1NF

B.2NF

C.3NF

D.BCNF

答案

参考答案:A

解析:[要点解析] 关系模式R在函数依赖范畴内主要有1NF、2NF、3NF、BCNF等4种范式。
如果关系模式R的每个关系的属性值都是不可分的数据项,则称该范式为第一范式(1NF)。1NF范式中存在非主属性对码的部分依赖。
如果关系模式R是1NF,且每个非主属性完全函数依赖于码(候选码),则称该范式为第二范式(2NF)。 2NF不允许有非主属性对码的部分函数依赖。
如果关系模式R是2NF,且每个非主属性都不传递依赖于R的候选码,则称该范式为第三范式(3NF)。 3NF不允许R的属性之间存在非平凡函数依赖。
如果关系模式R是1NF,且对于每个属性都不传递依赖于R的候选码,则称该范式为BCNF范式。
它们之间的相互关系是:1NF[*]2NF[*]3NF[*]BCNF。

阅读理解

阅读理解。

     Ben Jones was walking home from school one day when he saw a black cat. It was walking across the

srteet in front of him.Ben stopped, turned around, and walked down a different srteet to go home. Why did he

do that?

     "It's unluchy when a black cat crosses the street in front of you," says Ben, "I did not want to walk down

that street!"

     Many people think that some things bring bad luch of good luck. For example, some people think that 13

is an unlucky number. They never invite 13 people to a party.

     "I never walk under a ladder (梯子)," says Ben. "And I never open an umbrella inside the house. They both

bring bad luck."

     And what brings good luck? "We have a horseshoe over the front door of our house for good luck," says

Ben.  

     "And when I find a peney on the ground, I always pick it up. That brings me good luck all day!"

1. Why did Ben turn aroundd when he saw the black cat crossing the street?

A. Because he thought it was unlucky.

B. Because he disliked a black cat.

C. Because he was afraid of the cat.

2. In western countries,which number do some people think is unlucky?

A.  4.

B. 10.

C. 13.

3. Which of the following things can bring people good luck?

A. Opening an umbrella inside the house.

B. Walking under a ladder.

C. Picking up a penny on the ground.

4. Where does Ben's family have a horseshoe?

A. Over the front door.

B. Inside the house.

C. Under a ladder.

5. Which is the best title of the passage?

A. A black cat.

B. Good luck and bad luck.

C. Good things can bring good luck.

口语交际,情景问答题