2. 计算机
  3. A database system is a collection of interrelated data and a set of programs that allow users to access and modify these data．A

A database system is a collection of interrelated data and a set of programs that allow users to access and modify these data．A

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问题 A database system is a collection of interrelated data and a set of programs that allow users to access and modify these data．A major purpose of a database system is to provide users with an____①____ view ofthe data．That is，the system hides certain details of how the data are stored and maintained．For the system to be usable,it must retrieve data ____②____．The need for efficiency has led designers to use complex data ___③_____ to represent data in the database．Since many database—system users are not computer trained，developers hide the complexity from users through several levels of abstraction，to simplify users’interactions with the system．Physical level is the lowest level ofabstraction that describes ____④____ the data are actually stored．Logical level is the next-higher level of abstraction that describes___⑤_____data are stored in the database，and what relationships exist among those data．View level is the highest level of abstraction that describes only part of the entire database．
⑤
选项 A、how
B、what
C、which
D、when
答案B
解析 数据库系统由一组相互关联的数据和一组允许用户访问和修改这些数据的程序组成。数据库系统的主要目的是为用户提供数据的抽象视图。也就是说，系统隐藏了数据存储和维护的细节。为了使系统可用，它必须能够高效地检索数据。对效率的需求导致设计人员使用复杂的数据结构来表示数据库中的数据。由于许多数据库系统用户没有经过计算机培训，开发人员通过几个抽象层次向用户隐藏复杂性，以简化用户与系统的交互。物理层是描述数据实际存储方式的最底层抽象。逻辑层是描述数据库中存储哪些数据以及这些数据之间存在哪些关系的中间层抽象。视图层是仅描述整个数据库的一部分数据的最高层抽象。
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