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數據庫管理系統外文翻譯-管理系統.doc

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數據庫管理系統外文翻譯-管理系統.doc

大連交通大學2012屆本科生畢業設計論文外文文獻翻譯 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 數據庫管理系統 來源數據庫和網絡雜志 作者大衛安德森 眾所周知,數據庫是邏輯上相關的數據源集合。這些數據源可以按照不同的結構組織起來,以滿足單位和個人的多方面的要求。數據庫本身并沒有什么新東西早期的數據庫鑿在石頭上,記在名冊上,以及寫在索引卡中。而現在普遍記錄在可磁化的介質上,并且需要用計算機程序來執行必須的存儲和檢索操作。 處理與創建、訪問以及維護數據庫記錄有關的復雜任務的系統軟件包叫做數據管理。DBMS軟件包中的程序在數據庫及其用戶間建立接口(這些用戶可以是應用程序員)。 DBMS可組織、處理和顯示從數據庫選擇的數據源。該功能可以是決策者搜索、試探和查詢數據庫的內容,從而對正式報告中沒有的、不再出現的、且無計劃的問題做出回答。這些問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據庫知道獲得數據的答案。簡言之, DBMS將“管理”存儲的數據項,并從公共數據庫中匯集所需的數據項來響應那些非程序員的詢問。在面向文件的系統中,需要特定的用戶可以將它們的要求傳送給數據員,并準備信息。但是使用DBMS可為用戶提供一個更快的、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文件中的組織和結構,而DBMS可綜合多個文件的數據項,并回答某個用戶對信息的查詢。這意味著DBMS能夠訪問和檢索非關鍵字字段的數據,即 DBMS能夠將幾個大文件中邏輯相關的數據組織并聯在一起。 邏輯結構。確定這些邏輯關系是數據庫管理者的任務,由數據定義語言完成。DBMS在存儲、訪問和檢索操作過程中可以選用的邏輯技術有1表結構2層次(樹型)結構3網狀結構4關系性結構。 1.表結構。在該邏輯方式中,記錄通過指針連接在一起。指針是記錄中的一個數據項,它指出另一個邏輯相關的記錄的存儲位置。例如,顧客主文件中的記錄將包含每個顧客的姓名和地址,而且該文件中的每個記錄都由一個賬號標識。在記帳時期,顧客可以在不同時間購買許多東西。因此,公司保存一個發票文件來反應這些交易,這種情況下,可使用表結構顯示任意時間為支付的發票。顧客文件中的每個記錄都將包含這樣一個記錄,該鏈的最后一個記錄有一個作為指針的特殊字符標識。 2.層次結構。該邏輯方式中,數據單元的多級結構類似一棵倒立的樹,該樹 的樹根在頂部,而樹枝向下延伸。在層次結構中存在主從關系,唯一的根數據下是從屬的元或節點,而每個從屬的元或節點又一次“擁有”一個或多個元或節點。該結構中根下面的每個元或樹枝都只有一個所有者,這樣一個用戶擁有一個發票,而發票又有一個從屬。在樹型結構中,樹枝不能相連。 3.網狀結構。網狀結構不像樹型結構那樣不允許樹枝相連,它允許節點間多個方向相連。這樣每個節點都有幾個所有者,而它又可能有任意多個其它數據單元。數據管理軟件允許從文件的任意記錄開始提取該記錄中的所有信息。 4.關系型結構。關系型結構由許多表組成,數據則以關系的形式存儲在這些表中。例如,可以建立一些表將大學課程與該課程的老師以及上課的地點連接起來。為找到英語課的上課地點和教師名,現查詢課程/教師關系得到名字,然后再查詢課程/地點關系得到地點,當然也可以得到其它關系。這是一個新穎的數據組織技術,將來有望得到廣泛應用。 5.物理結構人們總是為了各自的目的,按邏輯方式設想或組織數據。這樣,在一個具體應用中,記錄R1和R2是邏輯相連且是按順序處理的。但是,在計算機系統中,這些在一個應用中邏輯上鄰接的記錄,物理上完全可能不存儲在一起。即錄在介質和硬件中的物理結構不僅取決于用戶定義的R1和R2中的數據的邏輯關系。例如,R1和R2可能是持有信用卡的用戶記錄而用戶要求每兩周將貨物運送到同一個城市的同一個街區。而從運輸部的管理規則看,R1和R2是按地理位置組織運輸記錄的順序項。都是在A/R應用中,可找到R1和R2表示的顧客,并可根據其完全不同的賬戶處理它們的賬目。簡言之,在許計算機化的信息記錄中,存儲記錄的物理單元,用戶是看不見的。 關于數據訪問技術的發展和最好的數據訪問技術(ADO) 在過去的五年里,微軟不斷推出開發人員所用的主要數據訪問技術,首先是Data Access Objects(DAO),然后是Remote Data Objects(RDO),現在是ActiveX Data Objects(ADO)。現在,新版本的ADO已經出現在微軟的WEB站點上。ADO的目標之一是簡化數據訪問。ADO建立在一些相當復雜的技術上,如OLE Db 和 ODBC(開放式數據庫連接),并且是為了能夠對存放于很多種不同類型的數據庫中的數據以編程方式進行訪問和修改而設計的。正是這種更為廣泛的訪問范圍,使ADO有別于以前的數據訪問技術。為了便于比較,我們首先簡單地看看ADO以前的技術DAO 和RDO。 數據訪問對象(DAO) DAO 最初是為了與Microsoft Access 數據庫進行交互而設計的。雖然可以使用DAO訪問SQL Server 和 Oracle 數據庫,但是很多開發人員都抱怨DAO應用于這些大型數據庫時性能太差。還有人抱怨DAO不允許程序員訪問SQL Server 和 Oracle 的一些更為珍貴的、功能更為前大的性能,例如存儲過程中的輸出和返回參數等。使用ADO處理Oracle 數據庫就相對自己進行腦部外科手術,不進行麻醉。是的,DAO的設計意圖是處理桌面數據庫的,而不是客戶機/服務器類型的數據庫。由于DAO的性能和訪問局限性令人感到沮喪,因此想要使

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