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数据库概况

2009-05-22 17:34

  New Words & Expressions:

  facilitate 使容易,促进
  retrieval n. 检索
  field n. 字段
  record 记录,
  alphabetically 按字母顺序地
  chronologically 按年代顺序排
  break down v. 分解
  build up 建造,装配,组成
  encyclopedia n. 百科全书
  reference n. 涉及,参考,引用
  bibliography 书目,参考书目
  periodical n. 期刊,杂志
  literature n. 文献,著作
  professional n. 专业人员,专家

  A database consists of a file or a set of files. The information in these files may be broken down into records, each of which consists of one or more fields. Fields are the basic units of data storage, and each field typically contains information pertaining to one aspect or attribute of the entity described by the database. Using keywords and various sorting commands, users can rapidly search, rearrange, group, and select the fields in many records to retrieve or create reports on particular aggregates of data.

  一个数据库由一个文件或文件集合组成。这些文件中的信息可分解成一个个记录,每个记录有一个或多个域。域是数据存储的基本单位,每个域一般含有由数据库描述的属于实体的一个方面或一个特性的信息。用户使用键盘和各种排序命令,能够快速查找、重排、分组并在查找的许多记录中选择相应的域,建立特定集上的报表。

  Database records and files must be organized to allow retrieval of the information. Early systems were arranged sequentially (i.e., alphabetically, numerically, or chronologically); the development of direct-access storage devices made possible random access to data via indexes. Queries are the main way users retrieve database information. Typically, the user provides a string of characters, and the computer searches the database for a corresponding sequence and provides the source materials in which those characters appear. A user can request, for example, all records in which the content of the field for a person's last name is the word Smith.

  数据库记录和文件的组织必须确保能对信息进行检索。早期的系统是顺序组织的(如:字母顺序、数字顺序或时间顺序);直接访问存储设备的研制成功使得通过索引随机访问数据成为可能。用户检索数据库信息的主要方法是query(查询)。通常情况下,用户提供一个字符串,计算机在数据库中寻找相应的字符序列,并且给出字符串在何处出现。比如,用户能够在所有记录中寻找所有last name域为Smith的记录。

  In flat databases [2], records are organized according to a simple list of entities; many simple databases for personal computers are flat in structure. The records in hierarchical databases are organized in a treelike structure, with each level of records branching off into a set of smaller categories. Unlike hierarchical databases, which provide single links between sets of records at different levels, network databases create multiple linkages between sets by placing links, or pointers, to one set of records in another; the speed and versatility of network databases have led to their wide use in business.

  在非结构化的数据库中,按照实体的一个简单列表组织记录;很多个人计算机的简易数据库是非结构的。层次型数据库按树型组织记录,每一层的记录分解成更小的属性集。层次型数据库在不同层的记录集之间提供一个单一链接,与此不同,网络型数据库在不同记录集之间提供多个链接,这是通过设置指向其它记录集的链或指针来实现的。网络型数据库的速度及多样性使其在企业中得到广泛应用。

  Relational databases are used where associations among files or records cannot be expressed by links; a simple flat list becomes one table, or “relation”, and multiple relations can be mathematically associated to yield desired information. Object-oriented databases store and manipulate more complex data structures, called “objects”, which are organized into hierarchical classes that may inherit properties from classes higher in the chain; this database structure is the most flexible and adaptable.

  当文件或记录间的关系不能用链表达时,使用关系型数据库。一个表或一个“关系”,就是一个简单的非结构列表。多个关系可通过数学关系提供所需信息。面向对象的数据库存储并处理更复杂的称为对象的数据结构,可组织成有层次的类,其中的每个类可以继承层次链中更高一级类的特性,这种数据库结构最灵活,最具适应性。

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