Computer English Unit 6 Database计算机英语第六单元 数据库.docx

1、Computer English Unit 6 Database计算机英语 第六单元 数据库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 organi

2、zation used its own file.在过去，一个机构中的每个应用程序都使用自己的文件。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

3、 financial aid to continue their education，the 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

4、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。Today, 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

5、definition：a database is a collection of related, 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 maintai

6、ns a database.数据库管理系统定义、创建和维护数据库。The DBMS also 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 s

7、ystem that allows access to data.硬件是指允许访问数据的计算机物理系统。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 da

8、ta.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 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 s

9、oftware that accesses it.在一个数据库中，数据是独立于对其进行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

10、by the organization should be kept together 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：e

11、nd users and application programs.最终用户和应用程序。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 o

12、f privileges and can control other users and 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 databa

13、se and has limited access.另一方面，普通用户只能使用数据库的一部分，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 pay

14、checks at the end of the month.例如，工薪发放应用程序需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 A

15、merican National Standards Institute Standards 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 Ar

16、chitecture数据库体系结构1. Internal Level1、内层The 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 interna

17、l 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 functions of the DBMS, such as queries，are also on this level.数据模型在该层定义，数据库管理系统的主要功能如查询也在该层

18、。The DBMS changes the internal view of data to the external view that users need to see.数据库管理系统把数据的内部视图转化为用户需要看到的外部视图。The conceptual level is an intermediary and frees users from dealing with the internal level.概念层是中介层，它使得用户不必与内层打交道。3. External Level3、外层The external level interacts directly with the

19、 user (end users or application programs).外层直接与用户（最终用户或应用程序）交互。It changes the data coming from the conceptual level to a format and view that is familiar to the users.它将来自概念层的数据转化为用户所熟悉的格式和视图。IV. Database Models四、数据库模型A database model defines the logical design of data.数据库模型定义数据的逻辑设计。The model also

20、describes the relationships between different parts of the data.它也描述数据的不同部分之间的关系。In the history of database design，three models have been in use:在数据库设计史上，使用过3种数据库模型：the hierarchical model, the network model，and the relational model.层次模型、网络模型和关系模型。1. Hierarchical Database Model1、层次数据库模型In the hierarc

21、hical model, data is organized as an inverted tree, each entity has only one parent but can have several children.在层次模型中，数据被组织成一棵倒置的树。每个实体只有一个父，但可有数个子。At the top of the hierarchy, there is one entity, which is called the root.在分层结构的顶部，有一个实体，称为根。Figure 6A-2 shows a logical view of an example of the h

22、ierarchical model.图6A-2给出了一个层次模型例子的逻辑视图。The hierarchical model is now obsolete.层次模型现在已经过时。Figure 6A-2:图6A-2：An Example of the Hierarchical Model Representing a University代表大学层级的一个层次模型实例2. Network Database Model2、网络数据库模型In the network model, the entities are organized in a graph, in which some entiti

23、es can be accessed through several paths (Figure 6A-3).在网络模型中，实体以图的形式来组织，图中的有些实体可通过多条路径访问（图6A-3）。There is no hierarchy.网络模型没有分层结构。This model is now also obsolete.这种模型现在也已经过时。Figure 6A-3:图6A-3：An Example of the Network Model Representing a University代表大学的一个网络模型实例3. Relational Database Model3、关系数据库模型I

24、n the relational model, data is organized in two-dimensional tables called relations.在关系模型中，数据被组织成称为关系的二维表。There is no hierarchical or network structure imposed on the data.关系模型没有分层或网络结构强加于数据。The tables or relations are, however, related to each other (Figure 6A-4).然而，表或关系是相互关联的（图6A-4）。Figure 6A-4:图

25、6A-4：An Example of the Relational Model Representing a University代表大学的一个关系模型实例The relational database management system (RDBMS) organizes the data so that its external view is a set of relations or tables.关系数据库管理系统组织数据，使其外部视图呈现为关系或表的集合。This does not mean that data is stored as tables:这并不意味着数据以表的形式存储

26、：the physical storage of the data is independent of the way in which the data is logically organized.数据的物理存储与数据的逻辑组织方式毫无关系。Figure 6A-5 shows an example of a relation.图6A-5给出了一个关系的例子。A relation in an RDBMS has the following features:关系数据库管理系统中的关系具有以下特征：Figure 6A-5:图6A-5：An Example of a Relation关系实例Na

27、me. 名称。Each relation in a relational database should have a name that is unique among other relations.关系数据库中的每个关系都应具有一个名称，而这个名称在所有关系中是独一无二的。Attributes. 属性。Each column in a relation is called an attribute.关系中的每一列都称为一个属性。The attributes are the column headings in the table in Figure 6A-5.在图6A-5的表中，属性是列

28、的标题。Each attribute gives meaning to the data stored under it.每个属性赋予存储在其下面的数据以意义。Each column in the table must have a name that is unique in the scope of the relation.表中的每一列都必须具有一个在关系的范围内独一无二的名称。The total number of attributes for a relation is called the degree of the relation.一个关系的属性总数称为该关系的度。For ex

29、ample, in Figure 6A-5, the relation has a degree of 3.例如，在图6A-5中，关系的度为3。Note that the attribute names are not stored in the database:注意属性名并不存储在数据库中：the conceptual level uses the attributes to give meaning to each column.概念层使用属性给每一列赋予一定的意义。n Tuples. 元组。Each row in a relation is called a tuple.关系中的每一行

30、称为一个元组。A tuple defines a collection of attribute values.元组定义一组属性值。The total number of rows in a relation is called the cardinality of the relation.一个关系中的总行数称为该关系的基数。Note that the cardinality of a relation changes when tuples are added of deleted.注意一个关系的基数随着元组的增加或删除而改变。This makes the database dynamic

31、.这使数据库具有了动态性。The relational model is one of the common models in use today.关系模型是今天使用的常见模型之一。The other two common models that are derived from the relational model are the distributed model and the object-oriented model.源自关系模型的另外两种常见模型是分布式模型和面向对象模型。4. Distributed Database Model4、分布式数据库模型The distributed database model is not a new model, but is based on the relational model.分布式数据库模型并非一种新模型，而是基于关系模型的。However, the data is stored on several computers that communicate through the Internet or a private wide area network.但是，数据存储在通过因特网或专用广域网通信的数台计算机上