问题 单项选择题

用户或应用程序看到的那部分的局部逻辑结构的描述是

A) 模式
B) 物理模式
C) 子模式
D) 内模式

答案

参考答案:C

解析: 数据库具有概念模式、外模式和内模式三级结构及外模式/模式映象、模式/内模式映象二层映象。概念模式是对数据库中全部数据的整体逻辑结构的描述,它由若干个概念记录类型组成。外模式又称子模式或用户模式,是用户与数据库的接口,是用户用到的那部分数据的描述,它由若干个外部记录类型组成。内模式是数据库在物理存储方面的描述,定义所有的内部记录类型、索引和文件的组成方式,以及数据控制方面的细节。外模式/模式映象存在于外部级和概念级之间,用于定义外模式和概念模式间的对立性,一般在外模式中描述,模式/内模式映象存在于概念级和内部级之间,用于定义概念模式和内模式间的对应性,一般在内模式中描述。

多项选择题
阅读理解

阅读理解

A

At three a.m., Jack Mills was sitting at the controls of the mail train. The train was made up of 13 cars. At the end of the train, 71 mailmen sat sorting the mail. Inside the second car, there were only five mailmen and 128 bags full of five-pound notes. This train had run more than 100 years without being robbed(抢劫). At three minutes past three, Mills and his helper, David Whitby, saw a yellow warning light. They slowed the train, and then stopped. Whitby went to the telephone beside the track. It was out of order. Then he saw a man moving between the second and the third cars. Before Whitby cjould give a warning, he was knocked down by two men. Mills’ cars with all the mailmen had been disconnected by the robbers. At the bridge, the bags of money were unloaded from the train and thrown into waiting trucks. One of the robbers who obviously knew the schedules(时刻表) of all the trains kept looking at his watch. At 3:45 he said, “That will have to be enough.” The robbers drove away with more than 2,500,000 pounds.

1. The robbery took place______.

A. before 3:03                                                 B. in the early morning 

C. after three o’clock in the afternoon         D. after 3:45

2. The robbers_____ before the train stopped.

A. were all in the train                              B. forced Mills to stop the train

C. were waiting for the train to stop            D. ordered Mills to go on driving

3. Why didn’t the other mailmen help the mailmen in the second car when the robbery happened?

A. Because they were busy sorting the mail.

B. Because they didn’t want to help the mailmen in the second car.

C. Because the last eleven cars were separated from the front cars.

D. Because they knew nothing about the robbery.

4. One of the robbers kept looking at his watch because________.

A. he wanted to know the exact time           B. he enjoyed looking at his watch

C. he didn’t know when another train would come

D. they had to leave before another train came

5. Which of the following is NOT true?

A. The train was out of order.             

B. The telephone was put out of order by the robbers

C. Whitby had an assistant driver.       

D. The robbers carried the money away by truck.