外文翻譯《數(shù)據(jù)庫系統(tǒng)》

1、Database systemA database may be defined as a collection of interrelated data stored together with as little redundancy as possible to serve one or more applications in an optimal fashion; the data are stored so that they are independents of programs which use the data; a common and controlled appro

2、ach 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 s

3、hould cause as little upheaval as possible the ease with which a database can be changed will have a major effect on the rate at whichdata-processing 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

4、 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 th

5、e 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 at

6、tribute 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.Logical data description is called a model.We must distinguish a record and a record examples, when talking about all p

7、ersonnel records when it is really a record type, not combined 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 ca

8、n get from one mode. T he 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 buffe

9、rs 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

10、“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 s

11、hould 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 o

12、n 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 1960s . the relational structure was introduced in the early 1970s.In the relation

13、al model two-dimensional tables represent the entities and their relationships every table represents an entities are represented by common 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

14、any complexities due to system-oriented 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 inter-face the model providers a relative

15、ly higher degree of data to make use of this property of the relational data model however, the design of the relations must be complete and accurate.Although some DBMS based on the relational data model are commercially available today it is difficult to provide a complete set of operational capabi

16、lities 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 tree-like structure made up of nodes and branches a node is a collection of da

17、ta 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 l

18、evel becomes 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 th

19、ere 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 database.Consists of a number of areas an area c

20、ontains 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 many-to-many relation-ship which occurs quite frequently in rap fife can be implemen

21、ted easily the network data model is very compel 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, according to some data model in the computer system in the organization

22、, storage and use of data collection linked to each other.數(shù)據(jù)庫系統(tǒng)一個數(shù)據(jù)庫可以被定義成一個相關(guān)數(shù)據(jù)的集合,這個集合盡可能小的冗余為一個或多個應(yīng)用程序在最理想的方式下服務(wù),存貯數(shù)據(jù)的目的是使他們與使用數(shù)據(jù)的程序獨立,一種相同的控制方法用于數(shù)據(jù)庫更新數(shù)據(jù)和修改,恢復(fù)已存在的數(shù)據(jù),如果一個系統(tǒng)在結(jié)構(gòu)上完全分離,則他們被稱為一個數(shù)據(jù)庫集合。大多數(shù)數(shù)據(jù)庫的最主要特點之一是他們需要不斷的更新和增加。當(dāng)數(shù)據(jù)類型和應(yīng)用被增加時,簡單重構(gòu)必須是可能的,重組織應(yīng)不需要重新寫應(yīng)用程序,總體上應(yīng)當(dāng)盡可能小的變化,這種不費力的改變數(shù)據(jù)庫將在數(shù)據(jù)處理的速度上產(chǎn)

23、生很大的影響。數(shù)據(jù)獨立這個術(shù)語經(jīng)常提出作為一個數(shù)據(jù)主要屬性之一,它是指數(shù)據(jù)和使用數(shù)據(jù)的應(yīng)用程序相互獨立,改變一項時不用改變另一項,當(dāng)一個單一的數(shù)據(jù)集合服務(wù)于不同的應(yīng)用時,不同的應(yīng)用程序感知數(shù)據(jù)項與記錄之間的管理說明相聯(lián)系,也和數(shù)據(jù)是怎樣存貯的和存在哪里有關(guān),在許多應(yīng)用程序的數(shù)據(jù)之間有很多種內(nèi)在的聯(lián)系。關(guān)于我們所存信息的項是一個實體,一個實體可以是一個實在的物體或抽象的事物,它有我們希望記錄的各種性質(zhì),它能描述一個真實的世界,數(shù)據(jù)項代表一個屬性,該屬性與有關(guān)的實體聯(lián)合。我們將屬性設(shè)計成值,一個屬性有一個特殊意義,據(jù)此而定義一個實體。邏輯數(shù)據(jù)庫的描述叫做一個模式。我們必須區(qū)別一個記錄和一個記錄的事

24、例,當(dāng)談?wù)摰饺w人員的記錄時它是一個真正的記錄類型,而不是與它相結(jié)合的數(shù)據(jù)值。一個模型用于描述數(shù)據(jù)庫中的所用存貯在數(shù)據(jù)庫中的數(shù)據(jù)項類型和記錄類型的總體圖表,子模式項指的是一個應(yīng)用程序員觀點下的數(shù)據(jù),許多不同的模式可以從一個模式中得到。模式和子模式是由數(shù)據(jù)庫管理系統(tǒng)使用的,數(shù)據(jù)庫管理系統(tǒng)的主要作用是執(zhí)行數(shù)據(jù)操作時服務(wù)于應(yīng)用程序。一個數(shù)據(jù)庫管理系統(tǒng)通常是用語同時處理多個數(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,并且很可能還包括使用物理模式實現(xiàn)概念模式的一些細節(jié),定義語言是一種高級語言,能夠讓你描述概念模型,術(shù)語為數(shù)據(jù)模型。數(shù)據(jù)模型選擇是很困難的,因為它必須有豐富的足夠結(jié)構(gòu)以描述真實世界有意義的各個方面,并且必須通過物理模式有能力、自動地決定概念模式的實現(xiàn),必須強調(diào),當(dāng)一個數(shù)據(jù)庫處理系統(tǒng)用于建立一個小數(shù)據(jù)庫中時,許多數(shù)據(jù)庫包括了上萬字節(jié),所以無效的執(zhí)行可能是災(zāi)難性的。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60年代已應(yīng)用于DBMS中,關(guān)系模型在20世紀70年代早期開始有所介紹