问题 单项选择题

通过()关系运算,可以从表1和表2获得表3;表3的主键为()。

表3的主键为()。

A.课程号

B.课程名

C.课程号、教师名

D.课程号、课程名

答案

参考答案:C

解析:

本题考查的是数据库关系运算方面的基础知识。自然连接是一种特殊的等值连接,它要求两个关系中进行比较的分量必须是相同的属性组,并且在结果集中将重复属性列去掉。一般连接是从关系的水平方向运算,而自然连接不仅要从关系的水平方向,而且要从关系的垂直方向运算。因为自然连接要去掉重复属性,如果没有重复属性,那么自然连接就转化为笛卡儿积。题中表1和表2具有相同的属性课程号,进行等值连接后,去掉重复属性列得到表3。若关系中的某一属性或属性组的值能唯一地标识一个元组,则称该属性或属性组为主键。从表3可见“课程号、教师名”才能决定表中的每一行,因此“课程号、教师名”是表3的主键。

阅读理解

判断正(T)误(F)。

     Fruit and vegetables should be a part of what we eat every day. Educating your children about the

importance of eating fruit and vegetables is important for their health. Here are some fun activities to

help you teach your children about fruit and vegetables.

     ★ Reading is one excellent way. Choose a colorful book with many different types of fruit and vegetables.

     ★ Provide your children with fruit and vegetable puzzles. Your children will be able to see what fruit and 

vegetables look like as they're putting the puzzles together. You can also teach them the different names of

fruit and vegetables.

     ★ Each day give your children different fruit and vegetables to eat. This will give them the chance to

taste many kinds of fruit and vegetables. Talk to your children about what they look like and how they taste

and smell.

     ★ Play fruit and vegetable games. Prepare little cups with fruit and vegetables cut up into small pieces.

Cover your children's eyes and give them one of the cups to try. After tasting, they have to guess what kind

of vegetable or fruit it is.

     ★ Grow vegetables. You can teach your children how vegetables grow by planting them. Your children

will also learn how to take care of vegetables.

( )1. Reading is a good way to help your children know about fruit and vegetables.                 

( )2. The puzzles can help children know how fruit and vegetables taste and smell.                

( )3. By playing fruit and vegetable games, children can know what fruit and vegetables look like.

( )4. Growing vegetables can teach your children how to look after vegetables.                    

( )5. The passage is mainly about the advantages of eating fruit and vegetables.                  

选择题