在关系模型中，表A中的某属性集是某表B的键，则称该属性集为A的__爱下问搜题

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问题填空题

在关系模型中，表A中的某属性集是某表B的键，则称该属性集为A的______。

答案

参考答案：外键或外码

解析：关系模型中的一个重要概念是键或码。键具有标识元组、建立元组间联系等重要作用。在二维表中凡能唯一标识元组的最小属性集称为该表的键或码。二维表中可能有若干个键，它们称为该表的候选码或候选键。从二维表的所有候选键中选取一个作为用户使用的键称为主键或主码。表 A中的某属性集是某表B的键，则称该属性集为A的外键或外码。

完形填空

Yesterday my grandfather told me about his early days.        he left school, he found a job in a post office. He worked hard there. Soon he had more money than his friends because of his hard work. It seemed that he would have a        future.
When he was 25 years old, however, he began to be        his job. He wanted a more exciting one, so he left the post office. Two months       , he found a new job in a big       . Workers in this company        traveled a lot. At first he really enjoyed the traveling. But after a year, he began to        it. He had to move around and he was always in a car or on a train. The        thing was that he couldn’t make any new friends. Even so, his boss was not satisfied with him. Two years later he        the company.
He was so        at that time that he sold his house. He had to accept a part-time job in a        shop. He was surprised to find that he loved it! He made new friends every day and he        reading books. Three years later, he        the shop owner's daughter. Soon they had their first child. In the next ten years, they had another three        and four more book shops! He became very rich indeed.
“Life is wonderful,” he told me. “The most        thing is that you should always try your best. Never give up!”

D．interesting

B．crazy about

C．interested in

问答题

一辆汽车朝山崖匀速行驶，在离山崖700米处鸣笛，汽车沿直线向前行驶40米后，司机刚好听到刚才鸣笛的回声．已知气温是15℃，求汽车的行驶速度？