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1、Unit 6 Database第六单元： 数据库 Section A:课文 A：Database Overview数据库概览I. Introduction一、引言 Data storage traditionally used individual, unrelated files, sometimes called flat files.数据存储传统上是使用单独的没有联系的文件，这些文件有时称为平面文件。In the past, each application program in an organization used its own file.在过去，一个机构中的每个应用程序都使用自
2、己的文件。In a university，for example, each department might have its own set of files:例如，在一个大学中，每个部门都可能有其自己的文件集：the record office kept a file about the student information and their grades, the financial aid office kept its own file about students that needed financial aid to continue their education，th
3、e scheduling office kept the names of the professors and the courses they were teaching, the payroll department kept its own file about the whole staff (including professors), and so on.档案办公室保存着关于学生信息和学生成绩的文件；经济资助办公室保存着其自己的关于需要经济资助以继续学业的学生的文件；调度办公室保存着教授的姓名和他们所教的课程；工薪发放部门保存着其自己的关于全体教职员工（包括教授）的文件，等等。T
4、oday, however, all of these flat files can be combined in a single entity, the database for the whole university.然而，所有这些平面文件今天都可结合成一个实体供整个大学使用的数据库。Although it is difficult to give a universally agreed definition of a database, we use the following common definition：a database is a collection of rela
5、ted, logically, coherent data used by the application programs in an organization.虽然难以给出一个普遍接受的数据库定义，但我们使用下面常见的定义： 一个数据库是被一个机构内的应用程序所使用的具有逻辑相干性的相关数据的集合。 Database Management Systems二、数据库管理系统 A database management system (DBMS) defines, creates, and maintains a database.数据库管理系统定义、创建和维护数据库。The DBMS als
6、o allows controlled access to data in the database.数据库管理系统也允许对数据库中的数据进行受控的访问。A DBMS is a combination of five components：hardware, software, data, users, and procedures.一个数据库管理系统由 5 个组成部分构成：硬件、软件、数据、用户和规程。1Hardware1、硬件 The hardware is the physical computer system that allows access to data.硬件是指允许访问数据
7、的计算机物理系统。For example，the terminals, hard disk，main computer and workstations are considered part of the hardware in a DBMS.例如，终端、硬盘、主机和工作站被认为是数据库管理系统的硬件组成部分。2Software2、软件 The software is the actual program that allows users to access，maintain, and update data.软件是指允许用户访问、维护和更新数据的实际程序。In addition, the
8、 software controls which user can access which parts of the data in the database.另外，软件还控制着哪个用户可以对数据库中的哪部分数据进行访问。3Data3、数据 The data in a database is stored physically on the storage device.数据库中的数据存储在物理存储设备上。In a database, data is a separate entity from the software that accesses it.在一个数据库中，数据是独立于对其进行
9、访问的软件的一个实体。This separation allows an organization to change the software without having to change the physical data or the way in which it is stored.这种独立使一个机构可以在不必更改物理数据及其存储方式的情况下更改软件。If an organization decides to use a DBMS，then all the information needed by the organization should be kept together
10、 as one entity, to be accessible by the software in the DBMS.如果一个机构决定使用一个数据库管理系统，那么该机构所需要的所有信息都应作为一个实体保存在一起，可由数据库管理系统中的软件访问。4. Users4、用户The term user in a DBMS has a broad meaning.在数据库管理系统中，用户这个术语有着广泛的定义。We can divide users into two categories:我们可以将用户分为两类：end users and application programs.最终用户和应用程序
11、。End users are those humans who can access the database directly to get information.最终用户是指可直接访问数据库以获取信息的人。There are two types of end users:最终用户又分为两类：database administrators (DBAs) and normal users.数据库管理员和普通用户。Database administrators have the maximum level of privileges and can control other users an
12、d their access to the DBMS, grant some of their privileges to somebody else, but retain the ability to revoke them at any time.数据库管理员拥有最高程度的特权，可以控制其他用户及其对数据库管理系统的访问，可以将其某些特权授予其他人并保留随时收回这些特权的能力。A normal user, on the other hand，can only use part of the database and has limited access.另一方面，普通用户只能使用数据库的
13、一部分，只能进行有限的访问。The other users of data in a database are application programs.数据库中数据的其他用户就是应用程序。Applications need to access and process data.应用程序需要访问和处理数据。For example, a payroll application program needs to access part of the data in a database to create paychecks at the end of the month.例如，工薪发放应用程序需
14、要在月底访问数据库中的部分数据，来开支付工薪的支票。5Procedures5、规程 The last component of a DBMS is a set of procedures or rules that should be clearly defined and followed by the users of the database.数据库管理系统的最后一个组成部分就是应该明确定义并为数据库用户所遵循的一套规程或规则。 . Database Architecture三、数据库体系结构The American National Standards Institute Standa
15、rds Planning and Requirements Committee (ANSI/SPARC) has established a three-level architecture for a DBMS:美国国家标准协会标准计划与需求委员会（ANSI/SPARC）为数据库管理系统确立了一个包含3 个层次的体系结构：internal, conceptual, and external (Figure 6A-1) .内层、概念层和外层（图 6A-1） 。Figure 6A-1:图 6A1：Database Architecture数据库体系结构1. Internal Level1、内层T
16、he internal level determines where data is actually stored on the storage devices.内层决定数据在存储设备上的实际存储位置。This level deals with low level access methods and how bytes are transferred to and from storage devices.该层涉及低级访问方法，以及字节如何传向和传自存储设备。In other words, the internal level interacts directly with the hardware.换句话说，内层直接与硬件交互。2. Conceptual Level2、概念层The conceptual level defines the logical view of the data.概念层定义数据的逻辑视图。The data model is defined on this level, and the main funct
