问题 单项选择题

关系数据库的任何检索操作都是由3种基本运算组合而成的,这3种基本运算不包括

A.联接

B.比较

C.选择

D.投影

答案

参考答案:B

解析:[分析] 关系代数中的运算分为两大类,一个是传统的集合运算,主要包括:并、交、差;而对于操作关系,关系代数中还提供了专门的运算,包括选择、投影、连接和自然连接。在这些运算中,5个基本的操作是:并、差、交、选择、投影。而将关系代数的运算运用到关系数据库中的检索操作中时,检索的基本操作是:连接、选择和投影。因此本题应该选B。

完形填空
阅读下面短文,掌握其大意,然后从11~25各题所给的A、B、C和D项中,选出最佳选项,并在答题卡上将该项涂黑。
eBooks still some way away from students
Twelve-year-old Tang Yin is one of the earliest students to test a digital textbook in China. The boy said he was a little  11  when he received the monochromatic(单色的) flat device in April, 2010, because he thought it should be a “   12   cool” laptop. But Tan still cherishes(珍爱) his new equipment like “protecting my  13 ,” said the fifth grader. After all, not every student has the opportunity to  14  the eBook, which is worth $294 (¥2,000 yuan), he said.
The 50 eBooks,   15  by Taiwan eBook manufacturer Chuanqi Photoelectric Technology in Yangzhou, Jiangsu province, enables one class at Yangzhou Sanyuanqiao Elementary School to be the   16  group of students in the Chinese mainland to replace their regular textbooks with   electronic ones. Out of 1000 students, Tang’s class was the lucky   17   .
“Our school could not   18  the project without the company’s donation,” said Yuan Shishan, vice-headmaster of the primary school.   19  the digital book is considered environmentally friendly, because it replaces paper books and saves money in the long run, the price is much higher than most Chinese   20  can afford. Also, some technical defects, the  21  of taking notes and monochromatic display, make the eBook fail to meet the expectations of students and teachers.
“There’s still a/an    22  way to go before the eBook really plays an important role in education,” said the headmaster.
According to Yuan, the annual average income of a   23   family in Yangzhou is about 20,000 to 30,000 yuan, and neither the parents nor the school can afford the 2,000 yuan  24 book.
The school doesn’t allow the students to take their eBooks to their homes. The devices are  25  after class, and are locked in the headmaster’s office.
小题1:
A.excitedB.interestedC.confused D.disappointed
小题2:
A.normalB.super C.light D.thin
小题3:
A.eyesB.gradesC.friendshipD.class
小题4:
A.buyB.giveC.useD.expect
小题5:
A.donatedB.manufacturedC.inventedD.designed
小题6:
A.originalB.firstC.secondD.final
小题7:
A.characters B.inspect ors C.recipients D.devotees
小题8:
A.supportB.introduceC.determineD.afford
小题9:
A.AfterB.As ifC.Now thatD.Although
小题10:
A.families B.classesC.schoolsD.provinces
小题11:
A.unfamiliarity B.inconvenienceC.discomfortD.dislike
小题12:
A.effectiveB.practical C.longD.narrow
小题13:
A.commonB.local C.smallD.general
小题14:
A.digitalB.regularC.newD.pleasant
小题15:
A.put onB.taken inC.looked onD.handed in
单项选择题