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数据库的基本概念
数据库和数据库技术给日益增长的计算机应用带来很大的推动。毫无疑问的,在所有计算机可使用的领域中,包括商业、工程、医疗、法律、教育和图书科学等等(仅列出这些),数据库都将起极重要的作用。“数据库”这个词是如此广泛地使用,以至我们必须着手去定义什么是数据库。我们的最初的定义是相当一般性的。
数据库是相关数据的集合。数据,我们是指已知的一些事实,这些事实是可以被记录并且有固有的意义。例如,考虑所有你知道的人的名字、电话号码和地址,你可以用索引地址薄记录这些数据,或者你可以用个人微机及诸如DBASEIII或LOTUS1-2-3这样的软件把他们记录在软磁盘上。这是一个相关并且具有内在意义的集合,因而它是一个数据库。
上面的数据库的定义是相当一般性的。例如,我们可以考虑所有的组成这页文字的词的集合,它们是相关的数据并因而是数据库。然而,作为通常使用的数据库术语常常是更有限制的,数据库有下面的内在的性质:
数据库是在逻辑上紧密相关的并且有某种内在意义的集合;
随即的数据聚合不能称为数据库;
数据库是为某种特定目的而设计、建造和聚集的数据,它有一些预期的用户和这些用户感兴趣的某些预想的应用;
数据库表示实际世界某一方面,有时称之为“微型世界”,对微型世界的改变反映在数据库中。
换言之,数据库有某种“源”,从这里数据被引导出来;由于真实世界的一些事件在某种程度上交换;有对数据库的内容非常关注的使用者。
数据库可以是任意规模的和有不同的复杂性。例如,名字和地址表在早期可能只涉及几百个记录,每个记录只有简单的结构。另一方面,一个大图书馆的卡片目录可以包含50万存放在不同分类目录中的卡片----这些分类可以是按主要作者的姓氏、按主题、按书的标题等等,每个分类都以字母表的顺序组织的。规模甚大而复杂的数据库可举美国国内税收局所设立的记录纳税人所填报的纳税表格的数据库。如果我们假设有1亿纳税人,每个纳税人平均填5个200个左右字符信息的表格,我们将得到一个100*(10)*200*5个字符(字节)信息的数据库。假设IRC(国内税收局)保存纳税人的除了当前的申报以外的过去的3个申报,我们将得到4*10字节的数据库。这样巨大的信息量必须有某种方式去组织和管理以使用户当必要时去搜索、检索和更新数据。
数据库可以由人工或机器生成和维护。当然,在这本书中我们主要兴趣在于计算机化的数据库。读书目录卡片是人工建立和维护的数据库的例子。计算机化的数据库可以是被一组专门为某个任务编写的应用程序或是被数据库管理系统所建立和维护的。
数据库管理系统(DBMS)是能使用户建立和维护数据库的程序集合。(DBMS)因而是一种有“通用目标”的软件系统,它可以使得为各种应用而定义、构造和操作数据库的过程变的容易。“定义”一个数据库包含规定在数据库中存储的数据类型以及每种数据类型的详细说明。“构造”数据库是在DBMS控制的某种存储介质上存储数据本身的过程。“操作”数据库包含这样的一些功能,诸如查询数据库以检索特定的数据,更新数据库以反映在微型世界中的变化,以及从数据中产生报告。
应当指出的是,实现计算机化的数据库不是必须使用“通讯目标”的DBMS软件。我们可以编写自己的一套建立和维护数据库的程序,实际上是建立我们自己“转用目标”的DBMS软件,不管在那种性能中----不论我们使用通用目标的DBMS,还是不是用它,我们通常都是要有处理数据库本身以外的相当大量的操作数据库的软件。数据库及其软件一起被称为数据库系统。
数据模型
数据库犯法的最基本特性之一是提供某种层次的数据抽象,这种抽象是掩蔽掉大多数数据库用户所不需要的数据存储的细节。数据模型是提供这种抽象的主要工具。数据模型是用于数据库结构的概念的集合。“数据库的结构”是指数据所具有的类型、联系和约束。大多数苏剧模型也包括在数据库中规定检索和更新的一些操作集合。
数据模型的分类
已经提出了许多数据模型。我们可以按照人们多提供的描述数据库结构的概念类型来分类数据模型。高层的或概念的数据模型提供一些很接近许多用户对数据的理解方式的概念,而低层的或物理的数据模型提供了一些描述数据如何在计算机内存储的细节的概念。低层数据模型所提供的概念一般地是为计算机专家使用的,不是为一般的终端用户使用的。在这两个极端之间是一类“实现”数据模型,这种模型提供一些可以被终端用户理解,但又可以以直接的方式在计算机系统上实现。
高层数据模型使用诸如入口、属性和联系等概念。入口是在数据库中表示的对象。属性是描述对象的某方面的性质。在对象间的联系是很容易在高层数据模型中表示的。这种数据模型有时叫做基于对象的模型,因为它们主要是描写对象和它们之间的相互联系。
“实现”数据模型在当前的商业的DBMS中是最经常使用的,并且包含三个最广泛使用的数据模型——关系的、网络的和层次的。它们用记录结构表示数据,并因而有时被叫做基于记录的数据模型。
物理数据模型用诸如记录格式、记录排序和存取路径等表示信息的方式描写数据是如何在计算机内存储的。存取路径是使对特定的数据库记录进行搜索更快的一种结构。
数据库管理系统的分类
用于分类DBMS的主要准则是DBMS所基于的“数据模型”。在当前商品化的DBMS中最常使用的数据模型是关系的、网络的和层次的模型。某些新近的DBMS是基于概念的或面向对象的模型。我们将把DBMS分成关系的、网络、层次的和其它的几类。
另一种准则用于分类DBMS是基于DBMS所支持的用户的数目。单用户系统一次只支持一个用户,而且大所数使用个人计算机。多用户系统包含了DBMS的大多数,能同时支持许多用户。
第三种准则是数据库分布的场地(或称节点----译注)数目。大多数DBMS是集中式的,它们的数据是存储在一个计算机场地。集中式的DBMS可以支持多个用户,但DBMS和数据库本身全部在单一的计算机场地。分布式的DBMS(DDBMS)可以有实际的数据库和由计算机网络连接起来的分散在许多场地的DBMS软件把它们连结起来。同构的DDBMS用同样的DBMS软件分散在多个场地。新近的趋势是开发软件能存取自治的预先存在异构DDBMS之下的数据库。这导致结盟DBMS(或多数据库系统)的产生。加入同盟的DBMS是松散耦合的而且有一定程度的局部自治。
我们也可以以可供存储文件的存取路径的类型的选择为基础来分类DBMS。一种著名的DBMS族是基于倒排的文件结构。
最后,DBMS可以是通用目标或专用目标的。当性能是作为首要考虑的因素时,可以设计和建立专用目标的DBMS,它看起来像文件,大多数关系数据库有高级查询语言,并支持有限形式的用户视图。
 
 
Database Systems
Fundamental concepts of Database
Databases and database technology are having a major impact on the growing use of computers. It is fair to say that databases will play a critical role in almost all areas where computers are used, including business, engineering, medicine, law, education, and library science, to name a few. The word ”database” is in such common use that we must begin by defining what a database is.
A database is a collection of related data. By data, we mean known facts that can be recorded and that have implicit meaning. For example, consider the names, telephone numbers, and addresses of all the people you know. You may have recorded this data in an indexed address book, or you may have stored it on a diskette using a personal computer and software such as DBASE Ⅲ or Lotus 1 – 2 – 3 . This is a collection of related data with an implicit meaning and hence is a database.
The above definition of database is quite general; for example, we may consider the collection of words that make up this page of text to be related data and hence a database. However, the common use of the term database is usually more restricted. A database has the following implicit properties:
• A database is a logically coherent collection of data with some inherent meaning. A random assortment of data cannot be referred to ad a database.
• A database is designed, built, and populated with data for a specific purpose. It has an intended group of users and some preconceived applications in which these users are interested.
• A database represents some aspect of the real world, sometimes called the mini-world.
Changes to the mini-world are reflected in the database.
In other world, a database has some source form which data are derived, some degree of interaction with events in the real world, and audience that is actively interested in the contents of the database.
A database can be of any size and of varying complexity. For example, the list of names and addresses referred to earlier may have only a couple of hundred records in it, each with a simple structure. On the other hand, the card catalog of a large library may contain half a million cards stored under different categories - by primary author’s last name, by subject, by book title, and the like - with each category organized in alphabetic order. A database of even greater size and complexity may be that maintained by the Internal Revenue Service to keep track of the tax forms filed by taxpayers of the United States. If we assume that there are 100 million taxpayers and each taxpayer files an average of five forms with approximately 200 characters of information per form, we would get a database of 100*(10)*200*5 characters (bytes) of information. Assuming the IRS keeps the past three returns for each taxpayer in addition to the current return, we would get a database of 4*(10) bytes. This huge amount of information must somehow be organized and managed so that users can search for, retrieve, and update the data as needed.
A database may be generated and maintained manually or by machine. Of course, in this book we are mainly interested in computerized database. The library card catalog is an example of a database that may be manually created and maintained. A computerized database may be created and maintained either by a group of application programs written specifically for that task or by a database management system.
A data base management system (DBMS) is a collection of programs that enables users to create and maintain a database. The DBMS is hence a general – purpose software system that facilitates the processes of defining, constructing, and manipulating databases for various applications. Defining a database involves specifying the types of data to be stored in the database, along with a detailed description of each type of data. Construction the database is the process of storing the data itself on some storage medium that is controlled by the DBME. Manipulating a database includes such functions as querying the database to retrieve specific data, updating the database to reflect changes in the mini-world, and generation reports from the data.
Note that it is not necessary to use general – purpose DBMS software for implementing a computerized database. We could write our own set of programs to create and maintain the database, in effect creating our own special – purpose DBMS software. In either case – whether we use a general – purpose DBMS or not – we usually have a considerable amount of software to manipulate the database in addition t the database itself. The database and software are together called a database system.
Data Models
One of the fundamental characteristics of the database approach is that is provides some level of data abstraction by hiding details of data storage that are not needed by most database users. A data model is the main tool for providing this abstraction. A data model is a set of concepts that can be used to describe the structure of a database. By structure of a database, we mean the data types, relationships, and constraints that should hold on the data. Most data models also include a set of operations for specifying retrievals and updates on the database.
Categories of Data Models
Many data models have been proposed. We can categorize data models based on the types of concepts they provide to describe the database structure. High – level or conceptual data models provide concepts that are close to the way many users perceive data, whereas low – level or physical data models provide concepts that describe the details of how data is stored in the computer. Concepts provided by low – level data models are generally meant for computer specialists, not for typical end users. Between these two extremes is a class of implementation data models, which provide concepts that may be understood by end users but that are not too for removed from the way data is organized within the computer. Implementation data models hide some details of data storage but can be implemented on a computer system in a direct way.
Implementation data models are the ones used most frequently in current commercial DBMS and include the three most widely used data models – relational, network, and hierarchical. They represent data using record structures and hence are sometimes called record – based data modes.
Physical data models describe how data is stored in the computer by representing information such as record formats, record orderings, and access paths. An access path is a structure that makes the search for particular database records much faster.
Classification of Database Management Systems
The main criterion used to classify DBMS is the data model on which the DBMS is based. The data models used most often in current commercial DBMS are the relational, network, and hierarchical models. Some recent DBMS are based on conceptual or object – oriented models. We will categorize DBMS as relational, hierarchical, and others.
Another criterion used to classify DBMS is the number of users supported by the DBMS. Single – user systems support only one user at a time, and are mostly used with personal computer. Multi-user systems include the majority of DBMS and support many users concurrently.
A third criterion is the number of sites over which the database is distributed. Most DBMS are centralized, meaning that their data is stored at a single computer site. A centralized DBMS can support multiple users, but the DBMS and database themselves reside totally at a single computer site. A distributed DBMS (DDBMS) can have the actual database and DBMS software distributed over many sites connected by a computer network. Homogeneous DBMS use the same DBMS software at multiple sites. A recent trend is to develop software to access several autonomous preexisting database stored under heterogeneous DBMS. This leads to a federated DBMS (or multi-database system), where the participating DBMS are loosely coupled and have a degree of local autonomy.
We can also classify a DBMS on the basis of the types of access path options available for storing files. One well – known family of DBMS is based on inverted file structures. Finally, a DBMS can be general purpose or special purpose. When performance is a prime consideration, a special – purpose DBMS can be designed and built for a specific application and cannot be used for other applications. Many airline reservations and telephone directory systems are special – purpose DBMS.
Let us briefly discuss the main criterion for classifying DBMS: the data mode. The relational data model represents a database as a collection of tables, which look like files. Most relational databases have high – level query languages and support a limited form of user views.
The network model represents data as record types and also represents a limited type of 1: N relationship, called a set type. The network model, also known as the CODASYL DBTG model, has an associated record – at – a – time language that must be embedded in a host programming language.
The hierarchical model represents data as hierarchical tree structures. Each hierarchy represents a number of related records. There is no standard language for the hierarchical model, although most hierarchical DBMS have record – at – a – time languages.
 

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