数据库管理系统外文翻译

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. there’s nothing new about data bases-early ones were 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 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 organize, 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 aren’t available in regular reports . these questions might initially 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 aren’t programmers. In a file-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 approaches 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 information. 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 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 operations: list structures,hierarchical(tree) structures ,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 each record in 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 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 the 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, these’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

4．Network structures. Unlike the tree approach, 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 needed