设关系R和S的元数分别为r和s。那么,由属于R但不属于S的元组组成的集合运算称为__

题型:单项选择题 共用题干题

问题:

设关系R和S的元数分别为r和s。那么,由属于R但不属于S的元组组成的集合运算称为__(1)__。在一个关系中找出所有满足某个条件的元组的运算称为__(2)__运算。对R和S进行__(3)__运算可得到一个r+s元的元组集合,其每个元组的前r个分量来自R的一个元组,后s个分量来自S的一个元组,如果R中有m个元组,S中有n个元组,则它们经__(4)__运算后共有__(5)__个元组。关系R和S的自然连接运算一般只用于R和S有公共__(6)__的情况。

空白(2)处应选择()

A.交

B.并

C.差

D.笛卡儿积

E.除

F.投影

G.选择

H.自然连接

考点:中级数据库系统工程师关系数据库基本理论关系数据库基本理论题库
题型:单项选择题 共用题干题

要约在()时生效。

A、发出

B、受要约人看到

C、到达受要约人

D、受要约人承诺

题型:单项选择题 共用题干题

After winning a big game, athletes are often asked how they feel. Usually athletes say how (1) it is to win this big game. On the contrary, the fact is that losing a big game, or in my case, all the games, can be even harder.

From the spectators’ point of view, last year’s basketball (2) for my high school team was nothing (3) of an embarrassment. And (4) 0 and 20 is certainly nothing to be proud of, that season had a bigger impact on me than any other. As a team captain, I knew it probably wouldn’t be the (5) year, but did I ever think we would lose every game Of course not. Since six of our top players had graduated, it was clear that we were a (n) (6) team who would struggle. The struggle began earlier than expected, (7) , as our team center was suspended (禁赛) for the season, and two key members decided to (8) for some reason. At this point, dropping out probably passed through every player’s mind, but in the end, we all stuck if out, (9) to work still harder.

Then there came a time when even our (10) and given up on us. Personally, I felt like it was (11) worthwhile giving my all. I thought, if even the coach doesn’t believe in us, why should I But just as my hope began to (12) , a teammate called a meeting. He said," Nobody thinks we are going to win, but, as teammates and friends, we (13) it to each other to go all out every game. "

It was that moment that (14) me how to be a leader. It hit me that I certainly didn’t (15) the role. As a leader you can never quit (16) the team who looks up to you. It is one (17) to be named captain, feel great and go through the motions, but it is quite another to be a real captain and make sure everyone realizes their full (18) all the time.

I am sure it is great to go through without losing and bringing home awards. But in all honesty, I (19) the fact that my team lost every game last year. It may not help me to become a better basketball player, but it already has made me a better leader, and (20)

4().

A.as

B.while

C.since

D.now that

题型:单项选择题 共用题干题

GDP是指()。

A.国民生产总值

B.国内生产总值

C.国民收入

D.国民生产净值

题型:单项选择题 共用题干题

消费者行为的基本目标是()

A.效用最大化

B.储蓄最大化

C.劳动供给最大化

D.边际效用达到零

题型:单项选择题 共用题干题

诊断和改正程序中错误的工作通常称为______。

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