题目内容

进行并、差、交集合运算的两个关系必须具有相同的______，即元组结构相同。

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散列法存储中处理碰撞的方法主要有两类：______和开地址法。

In the United States, newspaper reports tell of "crime in the streets" every day. (36) , all too often such stories have made many people think that (37) are so dangerous that they should be (38) . However, this is not necessarily the (39) . If an individual is careful wherever he goes, the (40) of having a misfortune in any large city are (41) small. Besides, the police have also given many tips to help people be on (42) against any possible attack. In general, when moving about a city at night, it is (43) to keep to the more traveled and better lighted streets. If you must travel through areas of a city that are considered dangerous, (44) . Avoid parks in the night since they are too likely to be deserted. Most cities have special telephone numbers to report a fire or to call the police. (45) . If the number is not posted, you can reach the police or fire department by calling the operator on the telephone. (46) .

清晰的结构是DBMS具有______的一个必要条件。

常用的模式分解的等价标准要求分解是具有______的和要求分解是保持函数依赖的。