Databa Management Systems
A databa (sometimes spelled data ba) is also called an electronic databa , referring to any collection of data, or information, that is specially organized for rapid arch and retrieval by a computer. Databas are structured to facilitate the storage, retrieval , modification, and deletion of data in conjunction with various data-processing operations .Databas can be stored on magnetic disk or tape, optical disk, or some other condary storage device.
A databa consists of a file or a t of files. The information in the files may be broken down into records, each of which consists of one or more fields. Fields are the basic units of data storage , and each field typically contains information pertaining to one aspect or attribute of the entity described by the databa . Using keywords and various sorting commands, urs can rapidly arch , rearrange, group, and lect the fields in many records to retrieve or create reports on particular aggregate of data.
Complex data relationships and linkages may be found in all but the simplest databas .Th
e system software package that handles the difficult tasks associated with creating ,accessing, and maintaining databa records is called a databa management system(DBMS).The programs in a DBMS package establish an interface between the databa itlf and the urs of the databa.. (The urs may be applications programmers, managers and others with information needs, and various OS programs.)
A DBMS can organize, process, and prent lected data elements form the databa. This capability enables decision makers to arch, probe, and query databa contents in order to extract answers to nonrecurring and unplanned questions that aren优美的风景图片’t available in regular reports. The questions might initially be vague and/or poorly defined ,but people can “brow如何下好象棋” through the databa until they have the needed information. In short, the DBMS will “manage” the stored data items and asmble the needed items from the common databa in respon to the queries of tho who aren’t programmers.
A databa management system (DBMS) is compod of three major parts:(1)a storage subsystem that stores and retrieves data in files;(2) a modeling and manipulation subsyst
em that provides the means with which to organize the data and to add , delete, maintain, and update the data;(3)and an interface between the DBMS and its urs. Several major trends are emerging that enhance the value and ufulness of databa management systems;
Managers: who require more up-to-data information to make effective decision
Customers: who demand increasingly sophisticated information rvices and more current information about the status of their orders, invoices, and accounts.
Urs: who find that they can develop custom applications with databa systems in a fraction of the time it takes to u traditional programming languages.
Organizations : that discover information has a strategic value; they utilize their databa systems to gain an edge over their competitors.
The Databa Model
A data model describes a way to structure and manipulate the data in a databa. The structural part of the model specifies how data should be reprented(such as tree, tables, and so on ).The manipulative part of the model specifies the operation with which to add, delete, display, maintain, print, arch, lect, sort and update the data.
好词好句好Hierarchical Model
T绩效目标怎么写信息加工he first databa management systems ud a hierarchical马字书法 model-that is-they arranged records into a tree structure. Some records are root records and all others have unique parent records. The structure of the tree is designed to reflect the order in which the data will be ud that is ,the record at the root of a tree will be accesd first, then records one level below the root ,and so on.
进qq空间The hierarchical model was developed becau hierarchical relationships are commonly found in business applications. As you have known, an organization char often describes a hierarchical relationship: top management is at the highest level, middle management at lower levels, and operational employees at the lowest levels. Note that within a strict hi
erarchy, each level of management may have many employees or levels of employees beneath it, but each employee has only one manager. Hierarchical data are 怀孕要吃什么characterized by this one-to-many relationship among data.
In the hierarchical approach, each relationship must be explicitly defined when the databa is created. Each record in a hierarchical databa can contain only one key field and only one relationship is allowed between any two fields. This can create a problem becau data do not always conform to such a strict hierarchy.
