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1、数据库（database，有时也可拼为data base）也可称为电子数据库,是为了方便计算机的快速查找和检索而专门组织起来的一组集合、数据和信息。由于各种数据处理操作命令的支持，数据库的数据存储、恢复、修改和删除得以简化。数据库可以被存储在磁盘、磁带、光盘或其它的一些二级存储设备上。 数据库由一个或一套文件组成。这些文件的信息可以被分解为多个记录，每个记录又包含一个或多个字段。字段是数据存储的基本单位，通常用来表示与数据库所描述的实体相关的某一属性信息。用户通过使用键盘和各种排序命令，能够快速查找、整理、分组和选择在许多记录中的字段，还可以检索某一类数据或创建报表。除了最简单的，所有数据库都
2、有复杂的数据关系和链接。用于处理和创建、访问和维护数据库记录相关任务的系统软件包被称为数据库管理系统（DBMS）。DBMS软件包中的程序可以在数据库和用户之间创建接口。（这些用户可以是应用程序员，管理员及其他需要信息的人员，和各种OS程序）一个DBMS可以组织、处理和表示从数据库中选出来的数据元。这个功能可以让决策者能够搜索、探索和查询数据库内容，从而获取到常规报告中不可用的、非经常性和无计划的答案。这些答案最初可能是模糊或者定义不清晰的，但是人们可以通过“浏览”数据库直到获取他们所需要的信息。简而言之，DBMS会“管理”存储的数据项，以及结合公共数据库中所需的数据项用以回答那些非程序员的查询
3、。DBMS由三个主要的部分组成：（1）一个用来存储和检索文件数据的存储子系统；（2）一个提供组织数据以及添加，删除，维护，更新数据的方法的建模和操作子系统；（3）还有一个是数据库和用户之间的接口。在提高数据库管理系统的价值和有效性方面正在展现以下一些重要发展趋势：管理人员：需要最新的信息以做出有效的决策。客户：需要越来越复杂的信息服务以及更多的有关其订单，发票和账号的当前信息。用户：发现他们可以使用传统的程序设计语言，在很短的一段时间内用数据库系统开发客户应用程序。商业公司：发现了信息的战略价值，他们利用数据库系统领先于竞争对手。数据库模型数据库模型描述了在数据库中结构化和操作数据的方法，模型
4、的结构部分描述了数据如何被描述（例如树、表等）：模型的操作部分规定了添加、删除、显示、维护、打印、查询、选择、分类和更新数据的操作。分层模型第一个数据库管理系统使用的是分层结构模型，该模型把记录设计成树形结构。一些记录是根记录，而其它所有记录则是唯一的父记录。树形结构的设计反应了哪些数据会被首先使用，树的根记录会被首先使用，然后是根节点下面的记录，等等。分层模型得以发展是因为分层关系在商业领域普遍存在。如你所知，一个组织结构图表通常描述了一种分层关系：高层管理人员在最高层，中层管理人员在更低的层次，负责具体事务的雇员在最底层。在一个严格的分层结构体系中，每个层次的管理者都有许多雇员或多个层次的
5、雇员，但是每个雇员只有一个管理者。分层结构就是以数据之间的一对多关系为特征的。在分层方法中，在创建数据库的时候每个关系必须得明确定义。在分层数据库中的每一记录只能包含一个关键字段，每个关系只被允许存在于两个字段之间。由于数据并不总是遵循这一严格分层结构关系，所以可能会出现一些问题。关系模型1970年，关于数据库的研究产生了一个重大突破，E. F. Codd提出了一种完全不同的方法来管理数据库。这种方法被称为关系模型，它使用了一个表作为数据结构。关系数据库是被最广泛使用的数据库结构。数据被组织到对应的表。每个表由被称为记录的行和被称为字段的列组成。每个记录包含了专用项目的字段值。例如，一张表中包
6、含了雇员的信息，一个记录则包含了数据的字段，比如一个人的姓、名和所居住的街道地址。结构化查询语言（SQL）是一种用于关系数据库中处理数据的查询语言。它是非程序性或者描述性语言，用户只须指定一种类似于英语的描述，用来确定操作，记录或描述记录组合。查询优化编辑器会把这些描述翻译成数据库的操作过程并执行。网状模型网络模型通过在数据之间创建一个链表结构，使得子记录可以链接到多个父记录。这种记录和链接结合在一起的方法，就是所谓的指针。它是指向一个记录存储位置的存储地址。使用网状方法，一个子记录可以链接到一个关键记录。同时，它本身也可以作为一个关键记录与其它一系列记录相链接。在历史上，网状模型比其它数据库
7、模型更具性能上的优势。但是在今天，这种优势的特点只有在大容量和高速处理过程中才是最重要的，比如自动柜员机网络，航空预定系统等。分层和网状数据库都是专用程序。如果开发一个新的应用程序，那么维护数据的一致性的是非常困难的。例如，假如开发一个新的退休金应用程序，虽然数据是相同的，但是必须创建一个新的数据库。对象模型最新的数据库管理方法是使用对象模型，记录由被称为对象的实体表示，对象既可以存储数据，又可以提供方法或程序执行特定的任务。对象模型使用的查询语言与开发数据库程序所使用的面向对象的程序设计语言是一样的，这就产生了问题，因为没有像SQL这样简单统一的查询语言。对象模型相对较新，只有很少的面向对象
8、数据库存在。但是对象模型已经引起关注，因为人们希望在选择开发面向对象程序的时候希望有一个基于面向对象的数据库。分布式数据库同样，分布式数据库是一种数据库的各个部分都分别存储在物理上相互分离的计算机上的数据库。分布式数据库的一个目标是访问数据时不考虑其存储的物理位置。要注意的是，当用户和数据分开时，通信和网络则开始发挥作用。分布式数据库需要常驻于大型主机上的软件，这些软件类似个人和大型机之间的桥梁，解决数据格式不兼容的问题。在理想情况下，大型主机上的数据库看起来像是一个大的信息仓库，而大部分处理则在个人计算机上完成。分布式数据库的一个缺点是它经常以主机中心模型为基础，大型主机看起来像个主人，而个
9、人计算机就好比一个奴隶。这个方法也有一些优势。通过集中控制下的数据库，前面提到的数据完整性和安全性的问题都得以解决。但是如今个人计算机、部门计算机，和分布式处理都需要计算机和计算机上之间以及应用程序之间在相等或对等的基础上进行相互通信。在数据库中，客户机/服务器模型提供用于分发数据库的框架。一种利用许多相互连接的计算机上运行的数据库应用程序的方法是把应用程序分解为相互独立的又相互合作的部分。客户端是一个最终用户或通过网络申请资源的计算机程序。服务器是一个运行着的计算机软件，存储着那些通过网络传输的申请。当申请的资源是数据库中的数据时，客户机/服务器模型则为分布式数据库提供了框架结构。文本服务器
10、是一种通过网络提供文件访问的软件。专用文本服务器是指一台指定作为文本服务器的计算机。这是有用的，例如，如果文件很大并且要求快速访问，在这种情况下，一台微型计算机或大型主机将会被用作文本服务器。分布式文件服务器把文件分散在多个，而不是单独一个计算机上。后一种文本服务器的优势包括存储和检索其它计算机上的文件，并且可以消除每台计算机上的重复文件。但是，这样会有一个很大的缺点。因为读写请求需要通过网络传播，所以在刷新文件的时候可能会产生问题。例如，两个用户同时请求同一个文件上的同一条记录并改变它时。解决这个问题的办法叫做记录锁定，这意味着在请求同一记录时，第二个请求必须得等到第一个请求完成时才可获取到
11、请求记录。同时，另外一些用户可以查看这个记录，但是同样不可以对这个记录做改变。数据库服务器是一个通过网络为数据库申请提供服务的软件例如，假设一个用户在他或她的个人计算机上输入一条数据查询指令，如果应用程序按照客户端/服务器模型设计，那么个人计算机上的查询语言通过网络传送数据库服务器上，并且当发现请求的数据时会发出通知。在工程界中可以找到许多分布式数据库系统的例子。如SUN公司的网络文件系统（NFS）被应用到计算机辅助工程应用程序中，将数据分散到由SUN工作站组成的网络上的不同硬盘之间。分布式数据库是一种革命性的进步，因为把数据放在它被使用的地方是最合理的。例如，一个大公司各个部门的计算机应该把
12、数据存放在本地，当公司的管理人员想整合部门数据时，这些数据应当允许被授权访问。DBMS软件会保护数据库的安全性和完整性，分布式数据库和非分布式数据库它们的用户而言将没有区别。Database Management Systems( 3th Edition ),Wiley ,2004, 5-12A introduction to Database Management SystemRaghu RamakrishnanA database (sometimes spelled data base) is also called an electronic database , referring
13、to any collection of data, or information, that is specially organized for rapid search and retrieval by a computer. Databases are structured to facilitate the storage, retrieval , modification, and deletion of data in conjunction with various data-processing operations .Databases can be stored on m
14、agnetic disk or tape, optical disk, or some other secondary storage device.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 typica
15、lly 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 aggregate of data.Compl
16、ex data relationships and linkages may be found in all but the simplest databases .The system software package that handles the difficult tasks associated with creating ,accessing, and maintaining database records is called a database management system(DBMS).The programs in a DBMS package establish
17、an interface between the database itself and the users of the database. (These users may be applications programmers, managers and others with information needs, and various OS programs.)A DBMS can organize, process, and present selected data elements form the database. This capability enables decis
18、ion makers to search, probe, and query database contents in order to extract answers to nonrecurring and unplanned questions that arent available in regular reports. These questions might initially be vague and/or poorly defined ,but people can “browse” through the database until they have the neede
19、d information. In short, the DBMS will “manage” the stored data items and assemble the needed items from the common database in response to the queries of those who arent programmers.A database management system (DBMS) is composed of three major parts:(1)a storage subsystem that stores and retrieves
20、 data in files;(2) a modeling and manipulation subsystem that provides the means with which to organize the data and to add , delete, maintain, and update the data;(3)and an interface between the DBMS and its users. Several major trends are emerging that enhance the value and usefulness of database
21、management systems;Managers: who require more up-to-data information to make effective decisionCustomers: who demand increasingly sophisticated information services and more current information about the status of their orders, invoices, and accounts.Users: who find that they can develop custom appl
22、ications with database systems in a fraction of the time it takes to use traditional programming languages.Organizations : that discover information has a strategic value; they utilize their database systems to gain an edge over their competitors.The Database ModelA data model describes a way to str
23、ucture and manipulate the data in a database. The structural part of the model specifies how data should be represented(such as tree, tables, and so on ).The manipulative part of the model specifies the operation with which to add, delete, display, maintain, print, search, select, sort and update th
24、e data.Hierarchical Model The first database management systems used a hierarchical model-that is-they arranged records into a tree structure. Some records are root records and all others have unique parent records. The structure of the tree is designed to reflect the order in which the data will be
25、 used that is ,the record at the root of a tree will be accessed first, then records one level below the root ,and so on.The hierarchical model was developed because hierarchical relationships are commonly found in business applications. As you have known, an organization char often describes a hier
26、archical relationship: top management is at the highest level, middle management at lower levels, and operational employees at the lowest levels. Note that within a strict hierarchy, each level of management may have many employees or levels of employees beneath it, but each employee has only one ma
27、nager. Hierarchical data are characterized by this one-to-many relationship among data.In the hierarchical approach, each relationship must be explicitly defined when the database is created. Each record in a hierarchical database can contain only one key field and only one relationship is allowed b
28、etween any two fields. This can create a problem because data do not always conform to such a strict hierarchy.Relational Model A major breakthrough in database research occurred in 1970 when E. F. Codd proposed a fundamentally different approach to database management called relational model ,which
29、 uses a table as its data structure.The relational database is the most widely used database structure. Data is organized into related tables. Each table is made up of rows called and columns called fields. Each record contains fields of data about some specific item. For example, in a table contain
30、ing information on employees, a record would contain fields of data such as a persons last name ,first name ,and street address.Structured query language(SQL)is a query language for manipulating data in a relational database .It is nonprocedural or declarative, in which the user need only specify an
31、 English-like description that specifies the operation and the described record or combination of records. A query optimizer translates the description into a procedure to perform the database manipulation.Network ModelThe network model creates relationships among data through a linked-list structur
32、e in which subordinate records can be linked to more than one parent record. This approach combines records with links, which are called pointers. The pointers are addresses that indicate the location of a record. With the network approach, a subordinate record can be linked to a key record and at t
33、he same time itself be a key record linked to other sets of subordinate records. The network mode historically has had a performance advantage over other database models. Today , such performance characteristics are only important in high-volume ,high-speed transaction processing such as automatic t
34、eller machine networks or airline reservation system.Both hierarchical and network databases are application specific. If a new application is developed ,maintaining the consistency of databases in different applications can be very difficult. For example, suppose a new pension application is develo
35、ped .The data are the same, but a new database must be created.Object Model The newest approach to database management uses an object model , in which records are represented by entities called objects that can both store data and provide methods or procedures to perform specific tasks.The query lan
36、guage used for the object model is the same object-oriented programming language used to develop the database application .This can create problems because there is no simple , uniform query language such as SQL . The object model is relatively new, and only a few examples of object-oriented databas
37、e exist. It has attracted attention because developers who choose an object-oriented programming language want a database based on an object-oriented model.Distributed DatabaseSimilarly , a distributed database is one in which different parts of the database reside on physically separated computers
38、. One goal of distributed databases is the access of information without regard to where the data might be stored. Keeping in mind that once the users and their data are separated , the communication and networking concepts come into play .Distributed databases require software that resides partiall
39、y in the larger computer. This software bridges the gap between personal and large computers and resolves the problems of incompatible data formats. Ideally, it would make the mainframe databases appear to be large libraries of information, with most of the processing accomplished on the personal co
40、mputer.A drawback to some distributed systems is that they are often based on what is called a mainframe-entire model , in which the larger host computer is seen as the master and the terminal or personal computer is seen as a slave. There are some advantages to this approach . With databases under
41、centralized control , many of the problems of data integrity that we mentioned earlier are solved . But todays personal computers, departmental computers, and distributed processing require computers and their applications to communicate with each other on a more equal or peer-to-peer basis. In a da
42、tabase, the client/server model provides the framework for distributing databases.One way to take advantage of many connected computers running database applications is to distribute the application into cooperating parts that are independent of one anther. A client is an end user or computer progra
43、m that requests resources across a network. A server is a computer running software that fulfills those requests across a network . When the resources are data in a database ,the client/server model provides the framework for distributing database.A file serve is software that provides access to fil
44、es across a network. A dedicated file server is a single computer dedicated to being a file server. This is useful ,for example ,if the files are large and require fast access .In such cases, a minicomputer or mainframe would be used as a file server. A distributed file server spreads the files arou
45、nd on individual computers instead of placing them on one dedicated computer.Advantages of the latter server include the ability to store and retrieve files on other computers and the elimination of duplicate files on each computer. A major disadvantage , however, is that individual read/write reque
46、sts are being moved across the network and problems can arise when updating files. Suppose a user requests a record from a file and changes it while another user requests the same record and changes it too. The solution to this problems called record locking, which means that the first request makes
47、 others requests wait until the first request is satisfied . Other users may be able to read the record, but they will not be able to change it .A database server is software that services requests to a database across a network. For example, suppose a user types in a query for data on his or her pe
48、rsonal computer . If the application is designed with the client/server model in mind ,the query language part on the personal computer simple sends the query across the network to the database server and requests to be notified when the data are found.Examples of distributed database systems can be
49、 found in the engineering world. Suns Network Filing System(NFS),for example, is used in computer-aided engineering applications to distribute data among the hard disks in a network of Sun workstation.Distributing databases is an evolutionary step because it is logical that data should exist at the location where they are being used . Departmental compu