问题 单项选择题

关系模型有3类完整性约束:实体完整性、参照完整性和用户定义的完整性。哪类(些)完整性是关系模型必须满足并由DBMS自动支持的 ( )

A.实体完整性

B.实体完整性和参照完整性

C.参照完整性和用户定义的完整性

D.实体完整性、参照完整性和用户定义的完整性

答案

参考答案:B

解析: 实体完整性和参照完整性是关系模型必须满足的完整性约束,必须由DBMS自动支持;而用户定义的完整性是应用领域需要遵循的约束,体现了具体领域中的语义约束,一般由DBMS或工具提供编写手段,由DBMS的完整性检查机制负责检查。故本题选择B。

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) .

14()

A.style

B.pattern

C.form

D.mold