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数据库管理系统外文翻译-管理系统.doc

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数据库管理系统外文翻译-管理系统.doc

大连交通大学2012届本科生毕业设计论文外文文献翻译 Database Management System SourceDatabase and Network Journal AuthorDavid Anderson You know that a data is a collection of logically related data elements that may be structured in various ways to meet the multiple processing and retrieval needs of organizations and individuals. There’s nothing new about data base-early ones were chiseled in stone, penned on scrolls, and written on index cards. But now database are commonly recorded on magnetically media, and computer programs are required to perform the necessary storage and retrieval operations. The system software package that handles the difficult tasks associated with created, accessing, and maintaining database records is in a DBMS package establish 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 programmers. A DBMS can organize, process, and present selected data elements from the database. This capability enables decision makers to search. Probe, and query data contents in order to extract answers to nonrecurring and unplanned questions that aren’t 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 needed 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 aren’t programmers. In a file-oriented system, users needing special information may communicate their needs to a programmers, who, when time permits, will information. The availability of a DBMS, however, offers users a much faster alternative communications patch see figure. Special, direct, and other file processing approaches ate used to organize and structure data in single files. But a DBMS is able to integrate data elements from several files to answer specific user inquiries fir information. This means that the DBMS is able to structure and tie together the logically related data from several large files. Logical structures. Identifying these logical relationships is a job of the data administrator. A data definition language is used for this purpose. The DBMS may then Employ one of the following logical structuring techniques during storage access, and retrieval operation list structures, hierarchical tree structures, and network structures, relational structures. 1. List structures. In this logical approach, records are linked together by the use of pointers. A pointer is a data item in one record that identifies the storage location of another logically related record. Records in a customer master file, for example, will contain the name and address of each customer, and an account number identifies each record in this file. During an accounting period, a customer may maintain an invoice file to reflect these transactions. A list structure could be used in this situation to show the unpaid invoices at any given time. Each in the customer file would point to the record location of the first invoice for that customer in the invoice file. This invoice record, in turn would be linked to later invoice for the customer. The last invoice in the chain would be identified by the use of a special character as a pointer. 2. Hierarchical structures. In this logical approach, data units are structured in multiple levels that graphically resemble an “upside down” tree with the root at the top and the branches formed below, there’s a superior-subordinate relationship in a hierarchical structure. Below the single-root data component are subordinate elements or one has only a single owner. Thus, as we see in figure, a customer owns an invoice, and the invoice has subordinate items. The branches in a tree structure are not connected. 3. Network structures. Unlike the tree approach, which dose not permit the connection of branches, the network structure permits the connection of the nodes in a multidirectional manner. Thus, each node may have several owners and may, in turn, own any number of other data units. Data, management software permits the extraction of the needed information from such a structure by beginning with any record in a file. 4. Relational structures. A relational structure is made up of many tables. The data are stored in the form of “relations” in these tables. For example, relation tables could be established to link a college course with the instructor of the course, and with the location of the in order to find the name of the instructor and the location of the English class, the course/instructor relation is searched to get the name, and the course/location relation is searched to get the class location. Many other relations are of course, possible. This is a relatively new database structuring approach that’s expected to be widely implemented in the future. 5. Physical structure. People visualize or structure data in logical ways for there Own purposes. Thus, records R1 and R2 may always be logically linked and processed in sequence in one particular application. However, in a computer system it’s quite possible that these records that are logically contiguous in one application are not physically stored together. Rather, the physical structure of the I/O and storage devices techniques used, but also on the different logical relationships that users may assign to the data found on R1 and R2. For example, R1 and R2 may be records of credit customers who have shipments send to the same block in the same city every two weeks. From the shipping department manager’s perspective, then, R1 and R2 are sequential entries on a geographically organized shipping report. But may be identified, and their accounts may be processed, according to their account numbers which are widely separated. In short, then the physical location of the stored records in many computer-based information systems is invisible to users. During the past five years, Microsoft has promoted Data Access Objects DAO, and then Remote Data Objects RDO, and now ActiveX Data Objects ADO as the primary data access technology for Visual Basic developers. It seems that Microsoft has been pushing a different data access technology with each successive version of Microsoft Visual Studio. Today, new versions of ADO are available on Microsofts Web site and ship with other products and technologies, such as Microsoft Windows 2000, Microsoft Windows NT 4 Service Packs, Microsoft Internet Explorer versions 3 and later, Microsoft SQL Server 6.5 Service Pack 5 and SQL Server 7, Microsoft Office 2000, and even Microsoft Expedia Streets Trips 2000. One of the goals of ADO is to simplify data access. ADO is built upon some fairly complex technologiesOLE DB and ODBC open database connectivityand is designed to allow you to programmatically access and modify data stored in a wide variety of databases. This broad reach is a departure from previous data access technologies. For the sake of comparison, lets take a quick glance at ADOs predecessors DAO and RDO. Data Access Objects DAO was originally designed to interact with Microsoft Access databases. Although you can use DAO to access SQL Server and Oracle databases, many developers complain about DAOs performance with these large database systems. Others complain that DAO doesnt permit programmers to access some of the richer, more powerful features of SQL Server and Oracle, such as output and return parameters on stored procedures. One of my coworkers likes to say that using DAO to work with an Oracle database is like performing brain surgery on youwithout anestheticswhile wearing oven mitts. Extreme Yesbut he does have a point. DAO is tuned to work with desktop databases, not client/server databases. Frustrated by DAOs performance and access limitations, developers who wanted to work with SQL Server and Oracle databases generally sought other options. Remote Data Objects Microsoft provided another option in RDO, which originally released with Visual Basic 4 Enterprise Edition. RDOs object model closely resembles the hierarchy of structures in the ODBC API. Programmers found that RDO provided much faster access to client/server database systems, such as SQL Server and Oracle, than DAO did. Although those familiar with the ODBC API quickly learned how to work with the RDO object model, developers lacking experience with that API, such as those who had been using DAO, found the RDO technology difficult to use. The object model itself wasnt the problem for most programmers learning RDO the nuances inherited from the ODBC API posed the greatest obstacles. Suddenly, programmers had to bone up on cursors and bookmarks. They had to learn many of the ins and outs of specific database systems. Does the error message The connection is busy with results from another hstmt ring any bells out there If you try to do the impossible on an ODBC connection to your database, RDO wont save you. Instead, youll get that error. DAO hid the problem from you by automatically creating another connection to your database to perform the action you requested. Another challenge that RDO posed for programmers accustomed to writing DAO code was that RDO lacked many of DAOs features, such as sorting, searching, and filtering. Other DAO functionality unavailable in the RDO world includes data definition language DDL interfaces to ODBC API functions such as Create Table and Create Field. Best of Both Worlds ActiveX Data Objects Programmers clamored for a data access technology that combined the simplicity and relative ease of use of DAO with the speed, power, and control of RDO. Initially introduced as part of the Microsoft Internet Information Server 3 package, ADO was intended to be all things to all people. Of course, such lofty goals are rarely fulfilled. While the initial release of ADO lacked many of Rod’s features, I believe that ADO 2.0 offered comparable functionality. Certain RDO features, such as mixed cursors, have yet to be implemented in ADO, but these features are few and far between. In fact, Im at a loss to name a single significant feature available in RDO that was not available in ADO 2.0 in one form or another. Im sure someone will tell me otherwise; a great way to find such features is to make a statement like that in a book like this. With the release of version 2.1, ADO and its supporting libraries began offering nearly all features available in DAO. DDL libraries were added to ADO in version 2.1 to provide functionality similar to functions available with DAO, such as Create Table, Create Field, and Create Index. Microsoft Jet and Replication Objects JRO in ADO 2.1 offers much of the Jet-specific functionality available via the DB Engine object in DAO. ADO 2.1 also added functionality to simplify the retrieval of newly generated identity values. ADO 2.5 adds no new functionality to more closely match the capabilities of DAO and RDO, because perhaps the only place where ADO lags behind DAO is in its searching and filtering capabilities. So ADO has most of the functionality of RDO and DAO as well as many helpful features not available in previous data access technologies. Database Management There are problems with traditional data management. A more subtle problem is data dependency. When a problem’s logic is tied to it’s physical data structure, changing that structure will almost certainly require changing the program. As a result, programs using traditional access methods can be difficult to maintain. The solution to both problems id often organizing the data as a single, integrated database. The task of controlling access to all the data can then be concentrated in a centralized database management system. How dose the use of a centralized database solve the data redundancy problem All data are collected and stored in a single place; consequently, there is one and only one copy of any given data element. When the value of an element an address, for example changes, the single database copy is corrected. Any program requiring access to this data element gets the same value, because there is only one value. How dose a database help to solve the data dependency problem Since the responsibility for accessing the physical data rests with the database management system, the programmer can ignore the physical data structure. As a result, programs tend to be much less dependent upon their data, and are generally much easier to maintain. Expect the trend toward database management to continue. 10 数据库管理系统 来源数据库和网络杂志 作者大卫安德森 众所周知,数据库是逻辑上相关的数据源集合。这些数据源可以按照不同的结构组织起来,以满足单位?#36879;?#20154;的多方面的要求。数据库本身并没有什么新东西早期的数据库凿在石头上,记在名册上,以及写在索引卡中。而现在普遍记录在可磁化的介质上,并且需要用计算机程序来执行必须的存储?#22270;?#32034;操作。 处理与创建、?#26790;?#20197;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管理。DBMS软件包中的程序在数据库及其用户间建立接口(这些用户可以是应用程序员)。 DBMS可组织、处理和显示从数据库选择的数据源。该功能可以是决策者搜索、试探和查询数据库的内容,从而对正式报告中没有的、不再出现的、且无计划的问题做出回答。这些问题最初可能是模糊的并且/或者是定义拙劣的,但是人们可以浏览数据库知道获得数据的答案。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来响应那些非程序员的询问。在面向文件的系统中,需要特定的用户可以将它们的要求传?#36879;?#25968;据员,并准备信息。但是使用DBMS可为用户提供一个更快的、用户可选择的通信方式。 顺序的直接的以及其它的文件处理方式通常用于单个文件中的组织和结构,而DBMS可综合多个文件的数据项,并回答某个用户对信息的查询。这意味着DBMS能?#29615;梦屎图?#32034;非关键字字段的数据,即 DBMS能够将几个大文件中逻辑相关的数据组织并联在一起。 逻辑结构。?#33539;?#36825;些逻辑关系是数据库管理者的任务,由数据定义语言完成。DBMS在存储、?#26790;屎图?#32034;操作过程中可以选用的逻辑技术有1表结构2层次(树型)结构3网状结构4关系性结构。 1.表结构。在该逻辑方式中,记录通过指针连接在一起。指针是记录中的一个数据项,它指出另一个逻辑相关的记录的存储位置。例如,顾客主文件中的记录将包含每个顾客的姓名和地址,而且该文件中的每个记?#32423;加?#19968;个账号标识。在记帐时期,顾客可以在不同时间购买许多东西。因此,公司保存一个发票文件来反应这些交易,这种情况下,可使用表结构显示任意时间为支付的发票。顾客文件中的每个记?#32423;?#23558;包含这样一个记录,该链的最后一个记录有一个作为指针的特殊字符标识。 2.层次结构。该逻辑方式中,数据单元的多级结构类似一棵倒立的树,该树 的树根在顶部,而树枝向下?#30001;臁?#22312;层次结构中存在主从关系,唯一的根数据下是?#37038;?#30340;元或节点,而每个?#37038;?#30340;元或节点又一次“拥有”一个或多个元或节点。该结构中根下面的每个元或树枝都只有一个所有者,这样一个用户拥有一个发?#20445;?#32780;发票又有一个?#37038;簟?#22312;树型结构中,树枝不能相连。 3.网状结构。网状结?#20849;?#20687;树型结构那样不允许树枝相连,它允许节点间多个方向相连。这样每个节点都有几个所有者,而它又可能有任意多个其它数据单元。数据管理软件允许从文件的任意记录开始提取该记录中的所有信息。 4.关系型结构。关系型结构由许多表组成,数据则以关系的?#38382;?#23384;储在这些表中。例如,可以建立一些表将大学课程与该课程的老师以?#21543;?#35838;的地点连接起来。为?#19994;接?#35821;课的上课地点和教师名,现查询课程/教师关系得到名字,然后再查询课程/地点关系得到地点,当然?#37096;?#20197;得到其它关系。这是一个新颖的数据组织技术,将来有望得到广泛应用。 5.物理结构人们总是为了各自的目的,按逻辑方式设想或组织数据。这样,在一个具体应用中,记录R1和R2是逻辑相连且是?#27492;?#24207;处理的。但是,在计算机系统中,这些在一个应用中逻辑上邻接的记录,物理上完全可能不存储在一起。即录在介质和硬件中的物理结?#20849;?#20165;取决于用户定义的R1和R2中的数据的逻辑关系。例如,R1和R2可能是持有信用卡的用户记?#32423;?#29992;户要求每两周将货物运送到同一个城市的同一个街区。而从运输部的管理规则看,R1和R2是按地理位置组织运输记录的顺序项。都是在A/R应用中,可?#19994;絉1和R2表?#38236;?#39038;客,并可根据其完全不同的账户处理它们的账目。简言之,在许计算机化的信息记录中,存储记录的物理单元,用户是看不见的。 关于数据?#26790;?#25216;术的发展和最好的数据?#26790;?#25216;术(ADO) 在过去的五年里,微软?#27426;?#25512;出开发人员所用的主要数据?#26790;?#25216;术,首先是Data Access Objects(DAO),然后是Remote Data Objects(RDO),现在是ActiveX Data Objects(ADO)。现在,新版本的ADO已经出现在微软的WEB?#38236;?#19978;。ADO的目标之一是简化数据?#26790;省DO建立在一些相当复杂的技术上,如OLE Db 和 ODBC(开放式数据库连接),并且是为了能?#27426;源?#25918;于很多种不同类型的数据库中的数据?#21592;?#31243;方式进行?#26790;?#21644;修改而设计的。正是这种更为广泛的?#26790;?#33539;围,使ADO有别于以前的数据?#26790;?#25216;术。为了便于比较,我们首先简单地看看ADO以前的技术DAO 和RDO。 数据?#26790;识?#35937;(DAO) DAO 最初是为了与Microsoft Access 数据库进行交互而设计的。虽然可以使用DAO?#26790;蔛QL Server 和 Oracle 数据库,但是很多开发人员都抱怨DAO应用于这些大型数据库时性能太差。还有人抱怨DAO不允许程序员?#26790;蔛QL Server 和 Oracle 的一些更为珍贵的、功能更为前大的性能,例如存储过程中的输出和返回参数?#21462;?#20351;用ADO处理Oracle 数据库就相对自己进?#24515;?#37096;外科手术,不进行麻醉。是的,DAO的设计意图是处理桌面数据库的,而不是客户机/服务器类型的数据库。由于DAO的性能和?#26790;示?#38480;性令人感到沮丧,因此想要使

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