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问题 单项选择题

关系R和S见表1和表2，关系代数表达式Ⅱ1.5

的结果为 (56) ，与该表达式等价的SQL语句为 (57) 。


答案

参考答案：B

解析： 本题表达式的含义是以R关系的C属性小于S关系的B属性作为连接条件，作R.A和S.B的投影运算。由于连接条件是R.C＜S.B，观察关系R第一行C属性为c，可以在关系S中找到第二行和第三行满足条件，于是得到结果(a,f)和(a,d)。同样方法，关系R第二行和第三行可得到结果(b,f)和(c,f)。第四行没有符合条件的结果。

单项选择题

针刺环跳穴时宜选

A．指切进针法
B．舒张进针法
C．提捏进针法
D．挟持进针法
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单项选择题

The United States is well known for its network of major highways designed to help a driver get from one place to another in the shortest possible time.Although these wide modem roads are generally (1) and well maintained, with few sharp curves and straight sections, a direct route is not always the most (2) one. Large highways often pass (3) scenic areas and interesting small towns. Furthermore, these highways generally (4) large urban centers, which means that they become crowded with heavy traffic during rush hours, (5) the" fast, direct" route becomes a very slow route.However,there is almost always another route to take if you are not in a hurry. Not far from the (6) new" superhighways", there are often older, (7) heavily traveled roads which go through the countrvside. (8) of these are good two-lane roads ; others are uneven roads curving through the country. These secondary routes may go up steep slopes, along high cliffs, or down frightening hillside to towns (9) in deep valleys. Through these less direct routes, longer and slower, they generally go to places (10) the air is clean and scenery is beautiful, and the driver may have a chance to get a fresh, clean view of the world.

第（10）空应选择（）

A．there

B．when

C．which

D．where
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