问题 单项选择题

表示实体类型及实体间联系的模型称为数据模型,其中关系模型主要特征是用 (43) 表示实体集, (44) 表示实体间联系。关系运算以关系代数为理论基础,关系代数的最基本操作是并、差、笛卡尔积、 (45) 。完整性规则提供了一种手段来保证当授权用户对数据库作修改时不会破坏数据的一致性。关系模型的完整性规则是对关系的某种约束条件,其中参照完整性要求 (46) 。假设关系R的属性F是关系S的外码,在对关系R和S的操作中,不可能破坏参照完整性的是 (47)

(45)处填()。

A.投影和自然连接

B.投影和选择

C.交和选择

D.交和自然连接

答案

参考答案:B

问答题 简答题
单项选择题

Writing about music is like dancing about architecture, or so the saying goes. Sometimes attributed to Frank Zappa, other times to Elvis Costello, this quote is usually intended to convey the futility of such an endeavor, if not the complete silliness of even attempting it. But Glenn Kurtz’s graceful memoir, Practicing: A Musician’s Return to Music, turns the expression on its head, giving it a different meaning by creating a lovely, unique book.
Kurtz picked up the guitar as a kid in a music-loving family, attended the Long Island music school, and went on to play on Merv Griffin’s TV show before graduating from Tufts University. Motivating the young Kurtz was the dream of reinventing classical guitar, as if by his great ambition alone he could push it from the margins of popular interest to center stage-something not even accomplished by the late Spanish guitarist Andres Segovia, perhaps the only artist of the form ever to reach anything resembling widespread celebrity.
This book reads like a love story of sorts: Boy meets guitar. Boy loves guitar. Guitar breaks boy’s heart or, more precisely, the ordinariness of a working musician’s life does so. "I’d just imagined the artist’s life naively, childishly, with too much longing, too much poetry and innocence and purity," Kurtz writes. "The guitar had been the instrument of my dreams. Now the dream was over. "
Boy leaves guitar. Were the story to end here, this book would be a tragedy, but after nearly a decade the boy returns to guitar, and although he has lost the enthusiasm he had in his youth, he finds his love of the guitar again in a way he never could have appreciated before.
Although Kurtz is writing about a unique musical path, his journey speaks eloquently to the heart of anyone who has ever desperately yearned to achieve something and felt the sting of disappointment. "Everyone who gives up a serious childhood dream—of becoming an artist, a doctor, an engineer, an athlete—lives the rest of their life with a sense of loss, with nagging what it is," he writes. "Is that time and effort, that talent and ambition, truly wasted \

As a young man Glenn Kurtz wanted to ______.

A.surpass Andres Segovia’s achievement

B.Transform classical guitar

C.become a TV music star

D.live on arts