外文翻譯-數(shù)據(jù)庫(kù)管理系統(tǒng)—.doc

大連交通大學(xué)2012屆本科生畢業(yè)設(shè)計(jì)(論文)外文文獻(xiàn)翻譯Database Management SystemSource：Database and Network Journal Author：David AndersonYou 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. Theres 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 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 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 arent 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 thenEmploy 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, theres 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 thats expected to be widely implemented in the future.5. Physical structure. People visualize or structure data in logical ways for thereOwn purposes. Thus, records R1 and R2 may always be logically linked and processed in sequence in one particular application. However, in a computer system its 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 managers 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 connectivity)and 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 ObjectsDAO 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 ObjectsMicrosoft 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 ObjectsProgrammers 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 Rods 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 ManagementThere are problems with traditional data management. A more subtle problem is data dependency. When a problems logic is tied to its 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數(shù)據(jù)庫(kù)管理系統(tǒng)來(lái)源：數(shù)據(jù)庫(kù)和網(wǎng)絡(luò)雜志作者：大衛(wèi)安德森眾所周知，數(shù)據(jù)庫(kù)是邏輯上相關(guān)的數(shù)據(jù)源集合。這些數(shù)據(jù)源可以按照不同的結(jié)構(gòu)組織起來(lái)，以滿足單位和個(gè)人的多方面的要求。數(shù)據(jù)庫(kù)本身并沒(méi)有什么新東西早期的數(shù)據(jù)庫(kù)鑿在石頭上，記在名冊(cè)上，以及寫在索引卡中。而現(xiàn)在普遍記錄在可磁化的介質(zhì)上，并且需要用計(jì)算機(jī)程序來(lái)執(zhí)行必須的存儲(chǔ)和檢索操作。處理與創(chuàng)建、訪問(wèn)以及維護(hù)數(shù)據(jù)庫(kù)記錄有關(guān)的復(fù)雜任務(wù)的系統(tǒng)軟件包叫做數(shù)據(jù)管理。DBMS軟件包中的程序在數(shù)據(jù)庫(kù)及其用戶間建立接口（這些用戶可以是應(yīng)用程序員）。DBMS可組織、處理和顯示從數(shù)據(jù)庫(kù)選擇的數(shù)據(jù)源。該功能可以是決策者搜索、試探和查詢數(shù)據(jù)庫(kù)的內(nèi)容，從而對(duì)正式報(bào)告中沒(méi)有的、不再出現(xiàn)的、且無(wú)計(jì)劃的問(wèn)題做出回答。這些問(wèn)題最初可能是模糊的并且/或者是定義拙劣的，但是人們可以瀏覽數(shù)據(jù)庫(kù)知道獲得數(shù)據(jù)的答案。簡(jiǎn)言之， DBMS將“管理”存儲(chǔ)的數(shù)據(jù)項(xiàng)，并從公共數(shù)據(jù)庫(kù)中匯集所需的數(shù)據(jù)項(xiàng)來(lái)響應(yīng)那些非程序員的詢問(wèn)。在面向文件的系統(tǒng)中，需要特定的用戶可以將它們的要求傳送給數(shù)據(jù)員，并準(zhǔn)備信息。但是使用DBMS可為用戶提供一個(gè)更快的、用戶可選擇的通信方式。順序的直接的以及其它的文件處理方式通常用于單個(gè)文件中的組織和結(jié)構(gòu)，而DBMS可綜合多個(gè)文件的數(shù)據(jù)項(xiàng)，并回答某個(gè)用戶對(duì)信息的查詢。這意味著DBMS能夠訪問(wèn)和檢索非關(guān)鍵字字段的數(shù)據(jù)，即 DBMS能夠?qū)讉€(gè)大文件中邏輯相關(guān)的數(shù)據(jù)組織并聯(lián)在一起。邏輯結(jié)構(gòu)。確定這些邏輯關(guān)系是數(shù)據(jù)庫(kù)管理者的任務(wù)，由數(shù)據(jù)定義語(yǔ)言完成。DBMS在存儲(chǔ)、訪問(wèn)和檢索操作過(guò)程中可以選用的邏輯技術(shù)有：1表結(jié)構(gòu)2層次（樹型）結(jié)構(gòu)3網(wǎng)狀結(jié)構(gòu)4關(guān)系性結(jié)構(gòu)。1.表結(jié)構(gòu)。在該邏輯方式中，記錄通過(guò)指針連接在一起。指針是記錄中的一個(gè)數(shù)據(jù)項(xiàng)，它指出另一個(gè)邏輯相關(guān)的記錄的存儲(chǔ)位置。例如，顧客主文件中的記錄將包含每個(gè)顧客的姓名和地址，而且該文件中的每個(gè)記錄都由一個(gè)賬號(hào)標(biāo)識(shí)。在記帳時(shí)期，顧客可以在不同時(shí)間購(gòu)買許多東西。因此，公司保存一個(gè)發(fā)票文件來(lái)反應(yīng)這些交易，這種情況下，可使用表結(jié)構(gòu)顯示任意時(shí)間為支付的發(fā)票。顧客文件中的每個(gè)記錄都將包含這樣一個(gè)記錄，該鏈的最后一個(gè)記錄有一個(gè)作為指針的特殊字符標(biāo)識(shí)。2.層次結(jié)構(gòu)。該邏輯方式中，數(shù)據(jù)單元的多級(jí)結(jié)構(gòu)類似一棵倒立的樹，該樹的樹根在頂部，而樹枝向下延伸。在層次結(jié)構(gòu)中存在主從關(guān)系，唯一的根數(shù)據(jù)下是從屬的元或節(jié)點(diǎn)，而每個(gè)從屬的元或節(jié)點(diǎn)又一次“擁有”一個(gè)或多個(gè)元或節(jié)點(diǎn)。該結(jié)構(gòu)中根下面的每個(gè)元或樹枝都只有一個(gè)所有者，這樣一個(gè)用戶擁有一個(gè)發(fā)票，而發(fā)票又有一個(gè)從屬。在樹型結(jié)構(gòu)中，樹枝不能相連。3.網(wǎng)狀結(jié)構(gòu)。網(wǎng)狀結(jié)構(gòu)不像樹型結(jié)構(gòu)那樣不允許樹枝相連，它允許節(jié)點(diǎn)間多個(gè)方向相連。這樣每個(gè)節(jié)點(diǎn)都有幾個(gè)所有者，而它又可能有任意多個(gè)其它數(shù)據(jù)單元。數(shù)據(jù)管理軟件允許從文件的任意記錄開(kāi)始提取該記錄中的所有信息。4.關(guān)系型結(jié)構(gòu)。關(guān)系型結(jié)構(gòu)由許多表組成，數(shù)據(jù)則以關(guān)系的形式存儲(chǔ)在這些表中。例如，可以建立一些表將大學(xué)課程與該課程的老師以及上課的地點(diǎn)連接起來(lái)。為找到英語(yǔ)課的上課地點(diǎn)和教師名，現(xiàn)查詢課程/教師關(guān)系得到名字，然后再查詢課程/地點(diǎn)關(guān)系得到地點(diǎn)，當(dāng)然也可以得到其它關(guān)系。這是一個(gè)新穎的數(shù)據(jù)組織技術(shù)，將來(lái)有望得到廣泛應(yīng)用。5.物理結(jié)構(gòu)：人們總是為了各自的目的，按邏輯方式設(shè)想或組織數(shù)據(jù)。這樣，在一個(gè)具體應(yīng)用中，記錄R1和R2是邏輯相連且是按順序處理的。但是，在計(jì)算機(jī)系統(tǒng)中，這些在一個(gè)應(yīng)用中邏輯上鄰接的記錄，物理上完全可能不存儲(chǔ)在一起。即錄在介質(zhì)和硬件中的物理結(jié)構(gòu)不僅取決于用戶定義的R1和R2中的數(shù)據(jù)的邏輯關(guān)系。例如，R1和R2可能是持有信用卡的用戶記錄而用戶要求每?jī)芍軐⒇浳镞\(yùn)送到同一個(gè)城市的同一個(gè)街區(qū)。而從運(yùn)輸部的管理規(guī)則看，R1和R2是按地理位置組織運(yùn)輸記錄的順序項(xiàng)。都是在A/R應(yīng)用中，可找到R1和R2表示的顧客，并可根據(jù)其完全不同的賬戶處理它們的賬目。簡(jiǎn)言之，在許計(jì)算機(jī)化的信息記錄中，存儲(chǔ)記錄的物理單元，用戶是看不見(jiàn)的。關(guān)于數(shù)據(jù)訪問(wèn)技術(shù)的發(fā)展和最好的數(shù)據(jù)訪問(wèn)技術(shù)（ADO）在過(guò)去的五年里，微軟不斷推出開(kāi)發(fā)人員所用的主要數(shù)據(jù)訪問(wèn)技術(shù)，首先是Data Access Objects（DAO），然后是Remote Data Objects（RDO），現(xiàn)在是ActiveX Data Objects（ADO）?，F(xiàn)在，新版本的ADO已經(jīng)出現(xiàn)在微軟的WEB站點(diǎn)上。ADO的目標(biāo)之一是簡(jiǎn)化數(shù)據(jù)訪問(wèn)。ADO建立在一些相當(dāng)復(fù)雜的技術(shù)上，如OLE Db 和 ODBC（開(kāi)放式數(shù)據(jù)庫(kù)連接），并且是為了能夠?qū)Υ娣庞诤芏喾N不同類型的數(shù)據(jù)庫(kù)中的數(shù)據(jù)以編程方式進(jìn)行訪問(wèn)和修改而設(shè)計(jì)的。正是這種更為廣泛的訪問(wèn)范圍，使ADO有別于以前的數(shù)據(jù)訪問(wèn)技術(shù)。為了便于比較，我們首先簡(jiǎn)單地看看ADO以前的技術(shù)：DAO 和RDO。數(shù)據(jù)訪問(wèn)對(duì)象（DAO） DAO 最初是為了與Microsoft Access 數(shù)據(jù)庫(kù)進(jìn)行交互而設(shè)計(jì)的。雖然可以使用DAO訪問(wèn)SQL Server 和 Oracle 數(shù)據(jù)庫(kù)，但是很多開(kāi)發(fā)人員都抱怨DAO應(yīng)用于這些大型數(shù)據(jù)庫(kù)時(shí)性能太差。還有人抱怨DAO不允許程序員訪問(wèn)SQL Server 和 Oracle 的一些更為珍貴的、功能更為前大的性能，例如存儲(chǔ)過(guò)程中的輸出和返回參數(shù)等。使用ADO處理Oracle 數(shù)據(jù)庫(kù)就相對(duì)自己進(jìn)行腦部外科手術(shù)，不進(jìn)行麻醉。是的，DAO的設(shè)計(jì)意圖是處理桌面數(shù)據(jù)庫(kù)的，而不是客戶機(jī)/服務(wù)器類型的數(shù)據(jù)庫(kù)。由于DAO的性能和訪問(wèn)局限性令人感到沮喪，因此想要使用S