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问题 填空题

关系数据库管理系统能实现的专门关系运算包括选择、连接和 【1】 。

答案

参考答案：投影

解析： 专门的关系运算主要包括对单个关系进行垂直分解(投影操作)或水平分解(选择操作)和对多个关系的结合(连接操作)等。

判断题

债券的偿还性、收益性、流动性和安全性之间具有相关性关系。（）
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单项选择题

The metric assigned to each network depends on the type of protocol. Some simple protocol, like RIP, treats each network as equals. The（1）of passing through each network is the same; it is one（2）count. So if a packet passes through 10 network to reach the destination, the total cost is 10 hop counts. Other protocols, such as OSPF, allow the administrator to assign a cost for passing through a network based on the type of service required.A（3）through a network can have different costs (metrics). For example, if maximum（4）is the desired type of service, a satellite link has a lower metric than a fiber-optic line. On the other hand, if minimum（5）is the desired type of service, a fiber-optic line has a lower metric than a satellite line. OSPF allow each router to have several routing table based on the required type of service.

1（）

A.cost

B.link-state

C.connection

D.diagram
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