【正文】 。下層的各個(gè)結(jié)點(diǎn)叫子結(jié)點(diǎn)。層次型數(shù)據(jù)模型是基于數(shù)型結(jié)構(gòu)的，由結(jié)點(diǎn)和數(shù)值組成，一個(gè)結(jié)點(diǎn)是描述此點(diǎn)上實(shí)體屬性的數(shù)據(jù)集合。數(shù)據(jù)模型在用戶截面上排出了存貯結(jié)構(gòu)和存取策略的細(xì)節(jié)，模型提供了一種高層次的數(shù)據(jù)關(guān)系，然而為了正確的使用關(guān)系數(shù)據(jù)模型，關(guān)系的設(shè)計(jì)必須是完備的和正確的。邏輯模式被定義為數(shù)據(jù)模式，帶有附加在其上的特別折數(shù)據(jù)庫管理系統(tǒng)的基本結(jié)構(gòu)，目前，數(shù)據(jù)庫管理系統(tǒng)有，三種主要的基本結(jié)構(gòu)，他們是：關(guān)系結(jié)構(gòu)，層次結(jié)構(gòu)，網(wǎng)絡(luò)結(jié)構(gòu)層次型和網(wǎng)絡(luò)型從20世紀(jì)60年代已應(yīng)用于DBMS中，關(guān)系模型在20世紀(jì)70年代早期開始有所介紹。一個(gè)數(shù)據(jù)庫管理系統(tǒng)通常是用語同時(shí)處理多個(gè)數(shù)據(jù)調(diào)用，它必須組織系統(tǒng)緩沖使不同的數(shù)據(jù)操作能夠一起處理，它提供種數(shù)據(jù)定義語言經(jīng)指定概念模式很可能還包括用物理模式實(shí)現(xiàn)概念模式的一些細(xì)節(jié)，定義語言是一種高級(jí)語言，能夠讓你描述概念模型，術(shù)語為數(shù)據(jù)模型。一個(gè)模型用于描述數(shù)據(jù)庫中的所用存貯在數(shù)據(jù)庫中的數(shù)據(jù)項(xiàng)類型和記錄類型的總體圖表，子模式項(xiàng)指的是一個(gè)應(yīng)用程序員觀點(diǎn)下的數(shù)據(jù)，許多不同的模式可以從一個(gè)模式中得到。屬性和他們之間的特殊的關(guān)系，他是一個(gè)合適的保存數(shù)據(jù)項(xiàng)的框架。邏輯數(shù)據(jù)庫的描述叫做一個(gè)模式。如果數(shù)據(jù)庫的作用僅僅是存貯數(shù)據(jù)，那它的結(jié)構(gòu)將是簡單的，大多數(shù)復(fù)雜性起因于數(shù)據(jù)必須所表現(xiàn)所存貯各種各樣的數(shù)據(jù)項(xiàng)之間的關(guān)系。主關(guān)鍵字是很重要的。主關(guān)鍵字定義為能唯一的確定一個(gè)記錄或一個(gè)元組的關(guān)鍵字。據(jù)此而定義一個(gè)實(shí)體。關(guān)于我們所存信息的項(xiàng)是實(shí)體，一個(gè)實(shí)體可以是一個(gè)實(shí)在的物體或抽象的事物，它有我們希望記錄的各種性質(zhì)，它能描述一個(gè)真實(shí)的世界，數(shù)據(jù)項(xiàng)代表一個(gè)屬性，該屬性與有關(guān)的實(shí)體聯(lián)合。當(dāng)數(shù)據(jù)類型和應(yīng)用被增加時(shí)，的簡單重構(gòu)必須是可能的，重組織應(yīng)當(dāng)不用重新寫應(yīng)用程序，總體上應(yīng)當(dāng)盡可能是小的變化，這種不費(fèi)力的改變數(shù)據(jù)庫將在數(shù)據(jù)處理的速度上產(chǎn)生很大的影響。一個(gè)數(shù)據(jù)庫呵以設(shè)計(jì)為批處理，實(shí)施處理或連機(jī)數(shù)據(jù)處理，一個(gè)數(shù)據(jù)庫系統(tǒng)包括應(yīng)用程序，數(shù)據(jù)庫管理系統(tǒng)和數(shù)據(jù)庫。 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.At abase may be designed for batch processing, realtimes processing, or inline processing. a data base system involves application program, DBMS and database.One o he 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 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 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 schemas 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 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 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 .networkThe hierarchical and network structures have been used for dams 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 datato make use of this property of the relational data model however, the design of the relations must be plete and accurate.Although some dams 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 dat