ic卡管理系統管理系統—計算機(論文)-資料下載頁

【導讀】建立和維護以及前端應用程序的開發兩個方面。對于前者要求建立起數據一致性。和完整性強、數據安全性好的庫。而對于后者則要求應用程序功能完備,易使用。代,不斷修正和改進,直到形成用戶滿意的可行系統。

　　

【正文】 most important characteristic of most databases is that they will constantly need to change and grow easy restructuring of the database must be possible as new data types and new applications are added. 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 anization 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 entities an entity may be a tangible object or no tangible if it has various properties which we may wish to record it can describe the real world the data item represents an attribute and the attribute must be associated which the relevant entity we relevant entity we design values to the attributes one attribute has a special significance in that it identifies the entity. The logical database description id referred to as a schema. The names of the entities and attributes and specifics the relations between them .It is a framework into which the values of the dataitems can be fitted. We must distinguish between a record type and a instance of the dataitem types and record types stored in a database the term subschema refers to an collocation programmer’s vies of the data he uses. Many different sub subschema can be derived from one schema. 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 dams will usually be handling multiple data calls concurrently, it must anize 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 schema it should be emphasized that while a dims 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 .work The hierarchical and work 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 accurate. Although 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 opint the highest node of the hierarchical tree structure is called a root the nodes at succeeding lower levels are called children. 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 work data model interconnects the entities of an enterprise into a work in the work data model a database. Consists 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 t