关于域关系演算和元组关系演算的区别，下列叙述中不正确的是 ()

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成人高等教育 / 数据库原理与应用

A、
不同之处是用域变量代替元组变量的每一个分量，下列
B、
与元组变量不同的是, 域变量的变化范围是某个值域而不是某一个关系
C、
可以像元组演算一样定义域演算的原子公式
D、
可以不像元组演算一样定义域演算的原子公式

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数据库原理与应用

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1[阅读题] Little Tommy was doing very badly in math. His parents had tried everything—tutors (家庭教师), cards, special learning centers—in short, everything they could think of. Finally they took Tommy to a catholic (天主教的) school. After the first day, little Tommy came home with a very serious look on his face. He didn’t kiss his mother hello. Instead, he went straight to his room and started studying. Books and papers were spread (铺开) out all over the room and little Tommy was hard at work. His mother was surprised. She called him down to dinner and as soon as he finished eating, he went back to his room, without a word. In no time he was back hitting the books as hard as before. This went on for some time, day after day while the mother tried to understand what was happening. Finally, little Tommy brought home his report card. He quietly put it on the table and went up to his room and hit the books. His mom looked at it and to her surprise, little Tommy got an A in math. She could no longer hold her curiosity (好奇心). She went to his room and asked, “Son, what was it? Was it the nuns (修女)?” Little Tommy looked at her and shook his head, “No. ” “Well then,” she asked again. “WHAT was it?” Little Tommy looked at her and said, “Well, on the first day of school, when I saw that man nailed (钉) to the plus sign (加号), I knew they weren’t joking. ”41. Why did Tommy’s parents send him to a catholic school? A. Because he could eat well there.B. Because he could earn more about nuns.C. Because his parents wanted him to do better in his math.D. Because his parents didn’t want him to learn math any more.42. Tommy’s mother felt surprised that his son ______.A. was still the same as usualB. ate so much at dinnerC. kissed her hello after schoolD. worked hard but said little43. “Hitting the books” means “______" in Chinese.A. 用功B. 捶书C. 发泄D. 振作44. The last sentence in the passage shows that ______.A. Tommy felt sorry for the mailB. Tommy was afraid of being nailedC. Tommy didn’t like the plus signD. Tommy liked playing jokes on others45. From the passage, we can infer (推断) that ____.A. teachers should be strict with their studentsB. mistaking (误解) might do good sometimesC. a catholic school is much better than other onesD. nuns are good at helping children with their math 大学英语