问题 单项选择题

在下列关系运算中,不改变关系表中的属性个数但能减少元组个数的是( )。

A) 并
B) 交
C) 投影
D) 笛卡尔乘积

答案

参考答案:B

解析: 本题考查数据库的关系运算。两个关系的并运算是指将第一个关系的元组加到第二个关系中,生成新的关系。因此,并运算不改变关系表中的属性个数,也不能减少元组个数。两个关系的交运算是包含同时出现在第一个和第二个关系中的元组的新关系。因此,交运算不改变关系表中的属性个数,但能减少元组个数。投影是一元关系操作。投影操作选取关系的某些属性,这个操作是对一个关系进行垂直分割,消去某些属性,并重新安排属性的顺序,再删除重复的元组。因此,投影运算既可以减少关系表中的属性个数,也可以减少元组个数。两个关系的笛卡尔乘积会增加属性个数,元组个数不变。

阅读理解

We provide summer English courses which are the best you can find. You will enjoy coming to Los Angeles to learn about American culture and improve your English at the same time. The courses last for four, six or eight weeks. They start at the beginning of July and August. Our teachers are well-trained and very experienced. They teach English classes for four hours a day, You can choose between small groups of two or three, or larger groups of up to 15. We provide weekly

tests to see the progress you’ re making with your language skills of reading, writing, speaking and listening. We also provide books and other materials.

As well as learning English, we want you to experience life in the USA. You live with an American family, and you take part in American life. You have meals with an American family and do some activities with them, too. For many of our students this is the most interesting part of the course. Many families create friendships with the students which last a long time. Of course, if you

prefer, we can arrange hotel accommodation.       

Of course, there are many things to do in Los Angeles. Every day we have activities which take place after class. You can choose to take trips to Hollywood, Disneyland or the famous beaches around LA. You can also go to various shopping centers, sports centers, movies and concerts. There are also weekend visits’  organized to San Francisco and other places of interest in California.

We try to give our students the best possible experience of English and life in the USA. Just ask our students! They say our summer English courses are fantastic!

We hope that you’ ll  enjoy a course with us, too. If you’ re interested, please fill in our application form where you will also find our list of prices.’ The course must be paid for one month before it begins.

小题1: The shortest summer English course will last for         

A.two weeks

B.four weeks

C.six weeks

D.eight weeks小题2:Which of the following is TRUE about the summer English courses?   

A.The students have to take an entrance test.

B.Each class has two or three groups of students.

C.All the teachers are young but very experienced.

D.Tests will be provided regularly to see if students have made progress.小题3: What is Paragraph 2 mainly about?

A.Accommodation and food.

B.Why the students enjoy themselves.

C.Sightseeing and other activities after class.

D.The most interesting part of the summer English courses.小题4:If you want to take part in the course, you must __ first.

A.pay for a one-month course

B.fill in the application form

C.choose an American family

D.learn about American culture小题5:The passage is a __

A.guide to Los Angeles

B.description of life in Los Angeles

C.description often English course

D.newspaper article on language learning

多项选择题