1、大连交通大学 2012 届本科生毕业设计 (论文 )外文文献翻译 1 Database Management System Source： Database and Network Journal Author： David 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 organization
2、s 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 syst
3、em 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 info
4、rmation 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 i
5、n 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 que
6、ries 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). Spe
7、cial, 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
8、 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 大连交通大学 2012 届本科生毕业设计 (论文 )外文文献翻译 2 then Employ one of the following logical structuring techniques
9、 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 th
10、e 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
11、 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 th
12、e 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 f
13、ormed 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 st
14、ructure 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
15、 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, re
16、lation 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
17、is searched to get the class location. Many other relations are of course, possible. This is a relatively new database structuring approach thats 大连交通大学 2012 届本科生毕业设计 (论文 )外文文献翻译 3 expected to be widely implemented in the future. 5. Physical structure. People visualize or structure data in logical w
18、ays 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 its quite possible that these records that are logically contiguous in one application are not physically stored together. Rather,
19、 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
20、 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 loca
21、tion 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 dev
22、elopers. 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 Windo
23、ws 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 technol
24、ogiesOLE 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 predece
25、ssors: 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,
