问题 单项选择题

设有关系R、S和T如下。关系T是由关系R和S经过哪种操作得到的
R

ABC
a1b15
a1b26
a2b38
a2b412
S
BE
b13
b27
b310
b32
b52
T
AR.BCS.BE
a1b15b27
a1b15b310
a1b26b27
a1b26b310
a2b38b310

答案

参考答案:C

解析: 选择是在关系R中选择满足给定条件的诸元组;投影是从R中选出若干属性列组成新的关系;连接(也称θ连接)是从两个关系的笛卡尔积中,选取它们属性间满足一定条件的元组。

阅读理解

The publication of "Harry Potter and the Half-Blood Prince" gladdens booksellers across China. The British and American editions were No. 1 and No. 3 respectively on the sales chart of the Beijing Xidan Book Building last week. The book's poster is highlighted and news about the book can often be heard on the radio.

Why is the book so attractive to children? With curious mood, the author got a copy of "Harry Potter". Originally, I wanted to glance over it and made some criticisms. But, out of expectation, the author has been deeply attracted by the magic world. On the other hand, one cannot help asking: where is  our own "Harry Potter"?

The Chinese nation has a history of 5,000 years, which cannot be regarded as short and its culture as not profound. With a vast market of youngsters, China did publish many books popular among children. However, why are the present works not as good as those imported?

There come two major reasons: Firstly, quite a number of children's books are of strong sense of teaching, and lack interest and entertainment. Children often have a feeling of being "educated". No wonder they don't like them. Secondly, children's reading materials involving "idiom stories" and "Chinese talents' stories", though always in different covers, are usually much the same or plagiarized. One knows the ending as early as in the beginning. In final analysis, the authors of the books seldom take children's requirements into consideration.

As a matter of fact, each child has his own wonderful imagination. They long to understand the world and nature. Similarly they have their own choices. They dislike similar stories, even if the stories are excellent. First-class reading for children should be very interesting, which contains knowledge and the essence of national culture, which are presented in children's language.

Market is a touchstone for products. The theory also holds true for cultural products. It is hoped that the authors of children's books can learn something from the good market of "Harry Potter" and write out more and better books loved by children.

64. The disadvantages of  our books are the following except_______.

A. they have little sense of interest and entertainment

B. many of  our books are of the same

C. the authors didn’t pay much attention to the children’s tastes.

D.  Our authors never know what our children need.

65. The reason why the imported book is so popular is that _______.

A. it has no sense of teaching

B. it opens a wonderful world for children to explore

C. it is only written in children’s language

D. it is advertised more on TV or poster.

66. What’s the feeling of the writer when he finishes reading “Harry Potter”?

A. He is so angry that he wants to criticize the book

B. He is also attracted by the book

C. He is shocked by the book, meanwhile he is worried about  our books.

D. He can’t help looking for Harry Potter.

67. What does the writer mean by writing “where’s  our own Harry Potter”?

A. We are short of books for children

B. We Chinese must produce books as popular as “Harry Potter”.

C. Authors should write books in the same way as “Harry Potter”

D.  Our authors are hoped to make the market of  our books for children take off

多项选择题