【文章內(nèi)容簡介】 plications in an optimal fashion。 the data are stored so that they are independents of programs which use the data。 a mon and controlled approach is used in adding new data and in modifying and retrieving existing data within the data base one system is said to contain a collection of databases if they are entirely separate in structure.The restructuring should be possible without having to rewrite the application program and in general should cause as little upheaval as possible the ease with which a database can be changed will have a major effect on the rate at which dataprocessing application can be developed in a corporation.The term data independence is often quoted as being one of the main attributes of a database int implies that the data and the may be changed without changing the other, when a single setoff data items serves a variety of applications, different application programs perceive different relationships between the data items,to a large extent database organization is concerned with the as how and where the data are stored.A database used for many applications can have multiple interconnection referred to as entity may be a tangible object or no tangible if it has various properties which we may wish to can describe the real data item represents an attribute and the attribute must be associated which the relevant relevant entity we design values to the attributes one attribute has a special significance in that it identifies the entity.Logical data description is called a model.We must distinguish a record and a record examples, when talking about all personnel records when it is really a record type, not bined with its data value.A model is used to describe the database used storage in the database data item type and record types of general charts, subschema paragraph refers to an application programmer view of data, many different patterns can get from one mode. The schema and the subschema are both used by the database management system the primary function of which is to serve the application programs by execution their data operations.A DBMS will usually be handling multiple data calls concurrently, it must organize its system buffers so that different data operations can be in process together, it provides a data definition language to specify the conceptual schema and most likely some of the details regarding the implementation of the conceptual schema by the physical schema the describe the conceptual schema in terms for a “data model”.The choice of a data model is a difficult one, since it must be such enough in structure to describe significant aspects of the real world, yet it must be possible to determine fairly automatically an efficient implementation of the conceptual conceptual schema by a physical should be emphasized that while a DBMS might be used to build small databases many databases involve millions of bytes and an inefficient implementation can be disastrous.Logical schemas are defined as data, pedals with the underlying structure of particular database management systems superimposed on them at the present time there are three main underlying structures for database management systems these are:relational,hierarchical,network.The hierarchical and network structures have been used for DBMS since the 1960’s . the relational structure was introduced in the early 1970’s.In the relational model twodimensional tables represent the entities and their relationships every table represents an entities are represented by mon columns containing values from a domain or range of possible values .The end user is presented with a simple data model his and her request and don not reflect any plexities due to systemoriented aspects a relational data model is what the user sees , but it is mot necessarily what will be implemented physically.The relational data model removes the details of storage structure and access strategy from the user interface the model providers a relatively higher degree of data to make use of this property of the relational data model however, the design of the relations must be plete and some DBMS based on the relational data model are mercially available today it is difficult to provide a plete set of operational capabilities with required efficiency on a large scale it appears today that technological improvements in providing faster and more reliable hardware may answer the question positively.The hierarchical data model is based no a treelike structure made up of nodes and branches a node is a collection of data attributes describing the entity at that opine the highest node of the hierarchical.A hierarchical data model always starts with a root node every node consists of one or more attributes describing the entity at that node dependent nodes can follow the succeeding levels the mode in the receding level bees the parent node of the new dependent nodes a parent node can have one child node as a dependent or many children nodes the major advantage of the hierarchical data model is the existence of proven database management systems that use the hierarchical data model as the basic structure there is a reduction of data dependency but any child mode is accessible only in a clumsy way this often results in a redundancy in stored data.The network data model interconnects the entities of an enterprise into a network in the network data model a of a number of areas an area contains records in turn a recode may consist of fields a set which is a grouping of records may reside in an area or span a number of areas a set type is based on the owner record type and the member record type the manytomany relationship which occurs quite frequently in rap fife can be implemented easily the network data model is very pel the application programmer must be familiar with the logical structure of the database.To meet the multiple users in a particular sector of the need for a variety of applications,