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1、A introduction to Database Management System Raghu RamakrishnanA database sometimes spelled data base is also called an electronic database referring to any collection of data or information that is specially organized for rapidsearch and retrieval by a computer. Databases are structured to facilita
2、te the storageretrieval modification and deletion of data in conjunction with various data-processingoperations .Databases can be stored on magnetic disk or tape optical disk or some othersecondary storage device.A database consists of a file or a set of files. The information in these files may be
3、brokendown into records each of which consists of one or more fields. Fields are the basic unitsof data storage and each field typically contains information pertaining to one aspector attribute of the entity described by the database . Using keywords and various sortingcommands users can rapidly se
4、arch rearrange group and select the fields in manyrecords to retrieve or create reports on particular aggregate of data.Complex data relationships and linkages may be found in all but the simplestdatabases .The system software package that handles the difficult tasks associated withcreating accessin
5、g and maintaining database records is called a database managementsystemDBMS.The programs in a DBMS package establish an interface between thedatabase itself and the users of the database. These users may be applicationsprogrammers managers and others with information needs and various OS programs.A
6、 DBMS can organize process and present selected data elements form the database.This capability enables decision makers to search probe and query database contents inorder to extract answers to nonrecurring and unplanned questions that arent availablein regular reports. These questions might initial
7、ly be vague and/or poorly defined butpeople can “browse” through the database until they have the needed information. Inshort the DBMS will “manage” the stored data items and assemble the needed itemsfrom the common database in response to the queries of those who arentprogrammers.A database managem
8、ent system DBMS is composed of three major parts:1a storagesubsystem that stores and retrieves data in files2 a modeling and manipulationsubsystem that provides the means with which to organize the data and to add deletemaintain and update the data3and an interface between the DBMS and its users.Sev
9、eral major trends are emerging that enhance the value and usefulness of databasemanagement systemsManagers: who require more up-to-data information to make effective decisionCustomers: who demand increasingly sophisticated information services and morecurrent information about the status of their or
10、ders invoices and accounts.Users: who find that they can develop custom applications with database systems in afraction of the time it takes to use traditional programming languages.Organizations : that discover information has a strategic value they utilize their databasesystems to gain an edge ove
11、r their competitors.The Database ModelA data model describes a way to structure and manipulate the data in a database. Thestructural part of the model specifies how data should be representedsuch as tree tablesand so on .The manipulative part of the model specifies the operation with which to adddel
12、ete display maintain print search select sort and update the data.Hierarchical ModelThe first database management systems used a hierarchical model-that is-they arrangedrecords into a tree structure. Some records are root records and all others have uniqueparent records. The structure of the tree is
13、 designed to reflect the order in which the datawill be used that is the record at the root of a tree will be accessed first then records onelevel below the root and so on.The hierarchical model was developed because hierarchical relationships are commonlyfound in business applications. As you have
14、known an organization char often describesa hierarchical relationship: top management is at the highest level middle managementat lower levels and operational employees at the lowest levels. Note that within a stricthierarchy each level of management may have many employees or levels of employeesben
15、eath it but each employee has only one manager. Hierarchical data are characterizedby this one-to-many relationship among data.In the hierarchical approach each relationship must be explicitly defined when thedatabase is created. Each record in a hierarchical database can contain only one key fielda
16、nd only one relationship is allowed between any two fields. This can create a problembecause data do not always conform to such a strict hierarchy.Relational ModelA major breakthrough in database research occurred in 1970 when E. F. Codd proposeda fundamentally different approach to database managem
17、ent called relationalmodel which uses a table as its data structure.The relational database is the most widely used database structure. Data is organized intorelated tables. Each table is made up of rows called and columns called fields. Eachrecord contains fields of data about some specific item. F
18、or example in a table containinginformation on employees a record would contain fields of data such as a persons lastname first name and street address.Structured query languageSQLis a query language for manipulating data in a relationaldatabase .It is nonprocedural or declarative in which the user
19、need only specify anEnglish-like description that specifies the operation and the described record orcombination of records. A query optimizer translates the description into a procedure toperform the database manipulation.Network ModelThe network model creates relationships among data through a lin
20、ked-list structure inwhich subordinate records can be linked to more than one parent record. This approachcombines records with links which are called pointers. The pointers are addresses thatindicate the location of a record. With the network approach a subordinate record can belinked to a key reco
21、rd and at the same time itself be a key record linked to other sets ofsubordinate records. The network mode historically has had a performance advantageover other database models. Today such performance characteristics are only importantin high-volume high-speed transaction processing such as automa
22、tic teller machinenetworks or airline reservation system.Both hierarchical and network databases are application specific. If a new application isdeveloped maintaining the consistency of databases in different applications can bevery difficult. For example suppose a new pension application is develo
23、ped .The data arethe same but a new database must be created.Object ModelThe newest approach to database management uses an object model in which recordsare represented by entities called objects that can both store data and provide methods orprocedures to perform specific tasks.The query language u
24、sed for the object model is the same object-oriented programminglanguage used to develop the database application .This can create problems becausethere is no simple uniform query language such as SQL . The object model is relativelynew and only a few examples of object-oriented database exist. It h
25、as attracted attentionbecause developers who choose an object-oriented programming language want adatabase based on an object-oriented model.Distributed DatabaseSimilarly a distributed database is one in which different parts of the database reside onphysically separated computers . One goal of dist
26、ributed databases is the access ofinformation without regard to where the data might be stored. Keeping in mind that oncethe users and their data are separated the communication and networking conceptscome into play .Distributed databases require software that resides partially in the larger compute
27、r. Thissoftware bridges the gap between personal and large computers and resolves theproblems of incompatible data formats. Ideally it would make the mainframe databasesappear to be large libraries of information with most of the processing accomplished onthe personal computer.A drawback to some dis
28、tributed systems is that they are often based on what is called amainframe-entire model in which the larger host computer is seen as the master and theterminal or personal computer is seen as a slave. There are some advantages to thisapproach . With databases under centralized control many of the pr
29、oblems of dataintegrity that we mentioned earlier are solved . But todays personal computersdepartmental computers and distributed processing require computers and theirapplications to communicate with each other on a more equal or peer-to-peer basis. In adatabase the client/server model provides th
30、e framework for distributing databases.One way to take advantage of many connected computers running databaseapplications is to distribute the application into cooperating parts that are independent ofone anther. A client is an end user or computer program that requests resources across anetwork. A
31、server is a computer running software that fulfills those requests across anetwork . When the resources are data in a database the client/server model provides theframework for distributing database.A file serve is software that provides access to files across a network. A dedicated fileserver is a
32、single computer dedicated to being a file server. This is useful for example ifthe files are large and require fast access .In such cases a minicomputer or mainframewould be used as a file server. A distributed file server spreads the files around onindividual computers instead of placing them on on
33、e dedicated computer.Advantages of the latter server include the ability to store and retrieve files on othercomputers and the elimination of duplicate files on each computer. A majordisadvantage however is that individual read/write requests are being moved acrossthe network and problems can arise
34、when updating files. Suppose a user requests arecord from a file and changes it while another user requests the same record andchanges it too. The solution to this problems called record locking which means that thefirst request makes others requests wait until the first request is satisfied . Other
35、 usersmay 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. Forexample suppose a user types in a query for data on his or her personal computer . If theapplication is designed with the client/server
36、 model in mind the query language part onthe personal computer simple sends the query across the network to the database serverand requests to be notified when the data are found.Examples of distributed database systems can be found in the engineering world. SunsNetwork Filing SystemNFSfor example i
37、s used in computer-aided engineeringapplications 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 existat the location where they are being used . Departmental computers within a largecorpora
38、tion for example should have data reside locally yet those data should beaccessible by authorized corporate management when they want to consolidatedepartmental data . DBMS software will protect the security and integrity of the database and the distributed database will appear to its users as no di
39、fferent from thenon-distributed database . 数据库管理系统的介绍 Raghu Ramakrishnan 数据库（database有时拼作 data base）又称为电子数据库，是专门组织起来的一组数据或信息，其目的是为了便于计算机快速查询及检索。数据库的结构是专门设计的，在各种数据处理操作命令的支持下，可以简化数据的存储，检索，修改和删除。数据库可以存储在磁盘，磁带，光盘或其他辅助存储设备上。 数据库由一个或一套文件组成， 其中的信息可以分解为记录，每一记录又包含一个或多个字段（或称为域）。字段是数据存取的基本单位。数据库用于描述实体，其中的一个字段通
40、常表示与实体的某一属性相关的信息。通过关键字以及各种分类（排序）命令，用户可以对多条记录的字段进行查询，重新整理，分组或选择，以实体对某一类数据的检索，也可以生成报表。 所有数据库（最简单的除外）中都有复杂的数据关系及其链接。处理与创建，访问以及维护数据库记录有关的复杂任务的系统软件包叫做数据库管理系统（DBMS） 。DBMS 软件包中的程序在数据库与其用户间建立接口。 （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操作系统程序）。 DBMS 可组织，处理和表示从数据库中选出的数据元。该功能使决策者能搜索，探查和查询数据库的内容，从而对在正规报告中没有的， 不再出现的且无法预料的
41、问题做出回答。这些问题最初可能是模糊的并且（或者）是定义不恰当的，但是人们可以浏览数据库直到获得所需的信息。简言之，DBMS 将“管理”存储的数据项，并从公共数据库中汇集所需的数据项以回答非程序员的询问。 DBMS 由 3 个主要部分组成： （1）存储子系统，用来存储和检索文件中的数据； （2）建模和操作子系统，提供组织数据以及添加，删除，维护，更新数据的方法； （3）用户和DBMS 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； 1.管理人员需要最新的信息以做出有效的决策。 2.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。 3.用
42、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。 4.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。数据库中数据的性质 1.数据整体性：数据库是一个单位或是一个应用领域的通用数据处理系统，他存储的是属于企业和事业部门、团体和个人的有关数据的集合。数据库中的数据是从全局观点出发建立的，他按一定的数据模型进行组织、描述和存储。其结构基于数据间的自然联系，从而可提供一切必要的存取路径，且数据不再针对某一应用，而是面向全组织，具有整体的结构化特征。 2.数据共享性：数据库中的数据是为众多用户所共享其信息而建立的，已经摆脱了具体程序的限制和制约。
43、不同的用户可以按各自的用法使用数据库中的数据；多个用户可以同时共享数据库中的数据资源，即不同的用户可以同时存取数据库中的同一个数据。数据共享性不仅满足了各用户对信息内容的要求，同时也满足了各用户之间信息通信的要求。数据管理的诞生 数据库的历史可以追溯到五十年前，那时的数据管理非常简单。通过大量的分类、比较和表格绘制的机器运行数百万穿孔卡片来进行数据的处理，其运行结果在纸上打印出来或者制成新的穿孔卡片。而数据管理就是对所有这些穿孔卡片进行物理的储存和处理。 1 然而， 9 5 1 年雷明顿兰德公司（Remington Rand Inc.）的一种叫做 UnivacI 的计算机推出了一种一秒钟可以输
44、入数百条记录的磁带驱动器，从而引发了数据管理的革命。1956 年 IBM 生产出第一个磁盘驱动器 the Model 305 RAMAC。此驱动器有 50 个盘片，每个盘片直径是 2 英尺，可以储存 5MB 的数据。使用磁盘最大的好处是可以随机地存取数据，而穿孔卡片和磁带只能顺序存取数据。 1951： Univac 系统使用磁带和穿孔卡片作为数据存储。 数据库系统的萌芽出现于 60 年代。当时计算机开始广泛地应用于数据管理，对数据的共享提出了越来越高的要求。传统的文件系统已经不能满足人们的需要。能够统一管理和共享数据的数据库管理系统（DBMS）应运而生。数据模型是数据库系统的核心和基础，各种
45、DBMS 软件都是基于某种数据模型的。所以通常也按照数据模型的特点将传统数据库系统分成网状数据库、层次数据库和关系数据库三类。 最早出现的是网状 DBMS，是美国通用电气公司 Bachman 等人在 1961 年开发成功的 IDS（Integrated DataStore）。1961 年通用电气公司（General ElectricCo.）的 Charles Bachman 成功地开发出世界上第一个网状 DBMS 也是第一个数据库管理系统 集成数据存储（Integrated DataStore IDS），奠定了网状数据库的基础，并在当时得到了广泛的发行和应用。IDS 具有数据模式和日志的特征。
46、但它只能在 GE主机上运行，并且数据库只有一个文件，数据库所有的表必须通过手工编码来生成。之后，通用电气公司一个客户BF Goodrich Chemical 公司最终不得不重写了整个系统。并将重写后的系统命名为集成数据管理系统（IDMS）。 网状数据库模型对于层次和非层次结构的事物都能比较自然的模拟，在关系数据库出现之前网状 DBMS 要比层次 DBMS 用得普遍。在数据库发展史上，网状数据库占有重要地位。 层次型 DBMS 是紧随网络型数据库而出现的。最著名最典型的层次数据库系统是 IBM 公司在 1968 年开发的 IMS （Information Management System） 一种适合其主机的层次数据库。 ， 这是 IBM公司研制的最早的大型数据库系统程序产品。从 60 年代末产生起，如今已经发展到IMSV6， N 提供群集、 路数据共享、消息队列共享等先进特性的支持。这个具有 3 0 年历史的数据库产品在如今的 WWW .
