1、附 录 ： 英 文 资 料 及 翻 译 英文原文： DATA BASE MANAGEMENT SYSTEMS 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. theres nothing new about data bases-early ones w
2、ere chiseled in st-one, penned on scrolls ,and written on index cards. but mow data bases are commonly recorded on magnetizeable media, and computer programs are required to perform the necessary storage ad retrieval operations. The system software package that handles the difficult tasks associated
3、 with creating , accessing ,and maintaining data base records is in a DBMS package establish an interface between the data base itself and the users of the data base . ( these users may be applications programmers ,managers and others with information needs , and various OS programs.) A DBMS can org
4、anize, process, and present selected data elements from the data base .this capability enables decision makers to search . probe ,and query data base contents in order to extract answers to nonrecurring and unplanned questions that arent available in regular reports . these questions might initially
5、 be vague and /or poorly defined ,but people can “browse” through the data base until they have the needed information . in short, the DBMS will “manage ”the stored data items and assemble the needed items from the common data base in response to the queries of those who arent programmers. In a file
6、-oriented system ,users needing special information may communicate their needs to a programmer , who ,when time permits, will information. The availability of a DBMS ,however, offers users a much faster alternative communications path (see figure). Sequential, direct, and other file processing appr
7、oaches are 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 integrate data elements from several files to answer specific user inquiries for inf
8、ormation. This means that the DBMS is able to access and retrieve data from nonkey record fields. That is, 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 administrato
9、r. 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 operations: list structures,hierarchical(tree) structures ,network structures, relational structures. 1 List structures. In this l
10、ogical 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 each record in
11、 this file is identified by an account number. During an accounting period, a customer may buy a number of items on different days . thus, the company 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
12、 time. Each record in 2 the customer file would contain a field that 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 invoices for the customer. The last invoice in the chain would be identified by t
13、he use of a special character as a pointer. 3 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, theses a superior-subordinate relationship in a hierar
14、chical 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 4 Network structures. Unlike the tree a
15、pproach, which does 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
16、needed information from such a structure by beginning with any record in a file. 5 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
17、the instructor of the course , and with the location of the .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 ate of cou
18、rse, possible. This is a relatively new data base structuring approach thats expected to be widely implemented in the future. Physical structures .people visualize or structure data in logical ways for their own purposes . thus , records R1 and R2 may always be logically linked and processed in sequ
19、ence 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 and techniques used, but also on the different l
20、ogical relationships that users may assign to the data found on R1 and R2 .for example, R1and R2 may be records of credit customers who have shipments send to the same block in the same city every 2 weeks . from the shipping department managers perspective , then ,R1and 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.
