问题 单项选择题

对关系数据模型而言,数据语义“每个学生有且仅有一个唯一的学号”属于( )。

A.域的完整性约束
B.实体完整性约束
C.引用完整性约束
D.一般完整性约束

答案

参考答案:B

解析: 在关系中用主关键字来唯一一标识一个实体,实体具有独立性,关系中的这种约束条件称为实体完整性。题目中的“每个学生有且仅有一个唯一的学号”就是主关键字来唯一标识一个实体,实体具有独立性。

阅读理解

阅读理解。

     The organization, Keep Time for Children, was started in 2002 to make people realize the importance of family time on weekends, particularly for families with school age children. __1__ We will try to help

parents take one day off on weekends to be with their family. __2__ What children want and need most

is to spend time with their parents, not with toys or taking part in activities. But with more and more people working on weekends, many children of school age don't see their parents anywhere near as often as they should. __3__ The movement is designed for families of all types and sizes under increasing pressure to

work on weekends. Because weekends are the very time when school age children are at home and in

need of their parents. We are going to put forward a Family Day Bill (议案) to the government. __4__ It

is aimed to solve the problems of increasing "shift (换班的) parenting" and of young children being left

"home alone" on weekends. __5__ If parents have a day off, they can spend a lot of time and energy with their children to share their happiness.

A. Keep Time for Children is now starting a movement.

B. The law is likely to make sure that parents take good care of their children.

C. Our aim is mainly to encourage parents to spend time with their children.

D. An American set up the organization.

E. It would give every parent of children under the age of 16 the right in law to a weekend day off each

week.

F. Our time is the most important thing that we can give our children.

G. Parents who do not share enough time with their children are to blame.

单项选择题