问题 单项选择题

对于“关系”的描述,正确的是( )。

A) 同一个关系中允许有完全相同的元组
B) 在一个关系中元组必须按关键字升序存放
C) 在一个关系中必须将关键字作为该关系的第一个属性
D) 同一个关系中不能出现相同的属性名

答案

参考答案:D

解析: 本题考查考生对关系的理解。选项A)、B)、C)都是错误的,同一个关系中,不允许有完全相同的元组,其元组的顺序是任意的,另外,关系的属性次序与是否为关键字无关,因此可以得出答案为选项D)。

单项选择题
单项选择题

Analysts have their go at humor, and I have read some of this interpretative literature, (1) without being greatly instructed. Humor can be (2) , (3) a frog can, but the thing dies in the process and the innards are (4) to any but the pure scientific mind.

One of the things (5) said about humorists is that they are really very sad ’people clowns with a breaking heart. There is some truth in it, but it is badly (6) . It would be more (7) , I think, to say that there is a deep vein of melancholy running through everyone’s life and that the humorist, perhaps more (8) of it than some others, compensates for it actively and (9) Humorists fatten on troubles. They have always made trouble (10) They struggle along with a good will and endure pain (11) , knowing how well it will (12) them in the sweet by and by. You find them wrestling with foreign languages, fighting folding ironing hoards and’ swollen drainpipes, suffering the terrible (13) of tight boots. They pour out their sorrows profitably, in a (14) of what is not quite fiction nor quite fact either. Beneath the sparking surface of these dilemmas flows the p (15) of human woe.

Practically everyone is a manic depressive of sorts, with his up moments and his down moments, and you certainly don’t have to be a humorist to (16) the sadness of situation and mood. But there is often a rather fine line between laughing and crying, and if a humorous piece of writing brings a person to the point (17) his emotional responses are untrustworthy and seem likely to break over into the opposite realm, it is (18) humor, like poetry, has an extra content, it plays (19) to the big hot fire which is Truth, and sometimes the reader feels the (20) .

3()

A.as

B.for

C.which

D.though