问题 单项选择题

第(38)~(40)题,基于“学生-选课-课程”数据库中的3个关系:
S(S#,SNAME,SEX,DEPARTMENT),主码是S#
C(C#,CNAME,TEACHER),主码是C#
SC(S#,C#,GRADE),主码是(S#,C#)

为了提高特定查询的速度,对SC关系创建唯一索引,应该创建在( )属性上。

A.(S#,C#)

B.(S#,GRADE)

C.(C#,GRADE)

D.GRADE

答案

参考答案:A

解析: 在一个关系中指定一个唯一标识该关系的元组,称为主码。可以通过查询主码来提高查询速度,一般都是在主码上建立索引,而关系SC的主码是(S#,C#),所以应该在(S#,C#)上建立唯一索引。

阅读理解

Have you ever wondered why birds sing? Maybe you thought that they were just happy.  After all,you probably sing or whistle when you are happy.

Some scientists believe that birds do sing some of the time just because they are happy. However,they sing most of the time for a very different reason. Their singing is actually a warning to other birds to stay out of their territory.

Do you know what a ’territory’ is? A territory is an area that an animal,usually the male,claims(声称)as its own. Only he and his family are welcome there. No other families of the same kind are welcome. Your garden and house are your territory where only your family and friends are welcome. If a stranger should enter your territory and threaten(威胁)you,you might shout. Probably this would be enough to frighten him away.

If so,you have actually scared the stranger away without having to fight him. A bird does the same thing. But he expects an outsider almost any time,especially at nesting(筑巢)season. So he is screaming all the time,whether he can see an outsider or not. This screaming is what we call a bird’s song,and it is usually enough to keep an outsider away.

Birds sing loudest in the spring when they are trying to attract a mate and warn others not to enter the territory of theirs..

You can see that birds have a language of their own. Most of it has to do with attracting mates and setting up territories.

小题1: Some scientists believe that most of the time bird’s singing is actually _______.

A.away of warning

B.an expression of happiness

C.an expression of anger

D.away of greeting小题2:What is a bird’s "territory"?.

A.A place where families of other kinds are not accepted.

B.A place where a bird may shut at the top of its voice

C.An area for which birds fight against each other.

D.An area which a bird considers to be its own.小题3: Why do birds keep on singing at nesting season?

A.Because they want to invite more friends.

B.Because they want to find outsiders around.

C.Because their singing helps frighten outsiders away.

D.Because their singing helps remove their fears.小题4:How does the writer explain bird’s singing?

A.By comparing birds with human beings.

B.By reporting experiment results.

C.By describing birds’ daily life.

D.By telling a bird’s story.

单项选择题