问题 单项选择题

在关系模式R<U>中,对于U的子集X和Y如果X→Y,且Y→X,则称Y对X的依赖为( )。

A.非平凡的函数依赖

B.完全函数依赖

C.传递函数依赖

D.部分函数依赖

答案

参考答案:A

解析:
这里要熟悉有关函数依赖的几个概念。
①函数依赖:设R(A1,A2,...,An]是一个关系模式。X和Y是{A1,A2..., An}的子集,若只要关系r是关系模式R的可能取值,则r中不可能有两个元组在X中的属性相等,而在Y中的属性值不等,则称X函数决定Y,记作X→Y。
②非平凡的函数依赖:若X→Y,但Y¢X,则称X→Y为非平凡的函数依赖。
③完全函数依赖:若X→Y,且对于X的任意一个真子集X都有

,则称Y对X完全函数依赖。
④部分函数依赖:若X→Y,但Y不完全函数依赖于X,则称Y对X部分函数依赖。
⑤传递函数依赖:若X→Y,(Y¢X),Y不函数依赖于X,Y函数决定Z,则称Z对X传递函数依赖。

阅读理解

阅读理解。

     English is fast becoming the language of science around the world, but what is its future among

everyday speakers? One expert points out that the percentage(比率) of native English speakers is

declining (降低) globally while the languages of other rapidly growing regions are being spoken by

increasing numbers of people. But English will continue to remain widespread and important.     

     However, British language scholar David Graddol says English will probably drop in dominance (主导) by the middle of this century to rank, after Chinese, about equally with Arabic, Hindi, and Urdu. "The

number of people speaking English as a first language continues to rise, but it isn't rising nearly as fast as the numbers of many other languages around the world, simply because the main population group has been

largely in the less developed countries where languages other than English have been spoken,' he says.    

     In a recent article in the journal Science, Mr. Graddol noted that three languages that are not near the

top of the list of the most widely spoken now might be there soon. These are Bengali, Tamil, and Malay,

spoken in south and Southeast Asia.     

     David Crystal, the author of the Cambridge Encyclopedia of the English Language, says about 1.5

billion of the world's six billion people speak it as a second tongue compared to the 400 million native

speakers. But another expert on the English language says Mr. Graddol underestimates (低估) the future

of its dominance. "Nobody quite knows what's going to happen because no language has been in this

position before. But all the evidence (证据) suggests that the English language snowball is rolling down a

hill and is getting faster,' he said. 

1. In David Graddol's opinion, English will _____.    

A. remain widespread and important    

B. be more important than any other language    

C. lose its dominant position    

D. die away in the near future

2. The underlined sentence in the last paragraph implies that _____.    

A. snowballs will roll down faster than language balls    

B. the English language snowball is rolling down faster than any time in the past    

C. English language will soon drop in dominance    

D. more and more language users will choose English

3. How many experts does the passage mention who express their ideas about the future of English?    

A. 2.            

B. 3.          

C. 4.            

D. 5.

4. According to David Graddol, which of the following will possibly be one of the languages that will be most widely spoken?     

A. Malay.    

B. Arabic.    

C. Urdu.        

D. German.

5. What should be the best title for the passage?    

A. English Remaining the Dominant Position    

B. The Future of English? Who Knows?    

C. Opinions from Different Experts    

D. The English Language Snowball Rolling Down

多项选择题