问题 单项选择题

对关系S和R进行集合运算,产生的元组属于S中的元组,但不属于R中的元组,这种集合运算称为( )。

A.并运算

B.交运算

C.差运算

D.积运算

答案

参考答案:C

解析:关系S和R的并运算是产生既包含S中的元组又包含R中的元组的集合。关系S和R的交运算是产生既属于S又属于R的元组的集合。关 系S和R的差运算产生属于S但不属于R的元组的集合。关系S和R的积运算产生的是两个关系的笛卡儿积。

单项选择题
完形填空

Everyone in our school loves sports. Every morning after we get up, we do morning exercises. After the second class, we do exercise again. (1)We only have P.E classes t_______ a week, b_______ we do exercise at five every afternoon. The most popular sport is basketball. The boys enjoy playing it and some of the girls like it, too .Another popular sport is football and in.every class there are a lot of football fans. We play volleyball when the weather is good. (2)We have school teams in basketball, football, and volleyball. Our teams often have friendship matches(友谊赛)with teams from other schools. When there is a match, many of us go to watch it and cheer for our side.(为我们队喝彩).

Besides(除了)ball games, some of us like track and field events(田径项目). We often practice running and jumping. Every term we have a sports meeting. Sports help us to keep fit and healthy(保持健康结实)。

小题1:在(1)处 填上适当的词,使句子通顺正确。

___________ ___________

小题2:把(2)处划线部分的句子译成中文。

______________________________________________________

小题3:回答问题:How often do we have a sports meeting?

_____________________________________________________.

小题4:在文中找出与所给句子意思相近的句子。

Both boys and some of the girls like playing it.

______________________________________________________.

小题5:在文中找出主题句,将其写在下面的横线上。

______________________________________________________.