在数据库系统中，我们把满足以下两个条件的基本层次联系的集合称为层次模型(33)在数据库系统中，将满足以下两个条件的基本层次联系集合称为网状模型(34)。

Ⅰ. 允许一个以上的结点无双亲  

Ⅱ. 有且仅有一个结点无双亲

Ⅲ．其它结点有且仅有一个双亲  

Ⅵ．有且仅有一个结点有多个双亲

Ⅴ. 有一个结点无双亲  

Ⅳ．一个结点可以有多于一个双亲

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In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

 How does the developer of a large database system achieve(71)and data independence? How does the developer make certain that stored items are(72)and do not simply disappear in the immensities of the database, as if in quicksand?

 One solution is to set up a(73)system, in which all data (the internal level), all records and files (the conceptual level), and all user interfaces (the external level) are controlled by one central programming group. This may seem the most useful solution. But worldwide enterprises such as the meteorological and satellite tracking networks, IBM, and General Motors gather and record information at a very large number of locations. To stay flexible and avoid top - heaviness, such an organization might want to avoid excessive centralization.

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In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

With the implementation of(66), the so - called network has become a reality. The provision of such facilities is the most important part of the network requirements. However, in many applications the communicating computers may be of different types. This means that they may use different programming languages and, more importantly, different forms of(67)interface between user(application) program, normally referred to, as application processes, and the underlying communication services may be(68). For example, one computer may be a small single - user computer, while another may be a large(69)system. In the earlier days of computer communication, these issues meant that only closed communities of computers (that is, from the same manufacturer) could communicate with each other in a meaningful way. IBM's Systems Network Architecture (SNA) and DEC's Digital Network Architecture (DNA) are just two examples of communication software packages produced by manufacturers to allow their systems to be interconnected together. These proprietary packages, however, of which there am still many inexistence, do not address the problem of universal interconnect ability, or open systems interconnection. In an attempt to alleviate this problem,(70), in the late 1970s, formulated a reference model to provide a common basis (or the coordination of standards developments and to allow existing and evolving standards activities to be placed into perspective with one another.

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