數(shù)據(jù)庫管理-外文翻譯-數(shù)據(jù)庫-閱讀頁
2025-02-08 11:57本頁面
【正文】 up of many tables. The data are stored in the form of “relations” in these tables. For example, relation tables could be established to link a college course with the instructor of the course, and with the location of the class. To find the name of the instructor and the location of the English class, the course/instructor relation is searched to get the name (“Fit”), and the course/location relation is a relatively new database structuring approach that’s expected to be widely implemented in the future. Physical Structures. People visualize or structure data in logical ways for their own purposes. Thus, records R1 and R2 may always be logically linked and processed in sequence in one particular application. However, in a puter system it’s quite possible that these records that are logically contiguous in one application are not physically stored together. Rather, the physical structure of the records in media and hardware may depend not only on the I/O and storage devices and techniques used, but also on the different logical relationships that users may assign to the data found in R1and R2. For example, R1 and R2 may be records of credit customers who have shipments send to the same block in the same city every 2 weeks. From the shipping department manager’s perspective, then, R1 and R2 are sequential entries on a geographically anized shipping report. But in the A/R application, the customers represented by R1 and R2 may be identified, and their accounts may be processed, according to their account numbers which are widely separated. In short, then, the physical location of the stored records in many puterbased information systems is invisible to users. 數(shù)據(jù)庫管理系統(tǒng)的組織技術(shù) 順序的、直接的以及其他的文件處理方式常用于單個文件中數(shù)據(jù)的組織和構(gòu)造，而 DBMS可綜 合幾個文件的數(shù)據(jù)項以回答用戶對信息的查詢，這就意味著 DBMS能夠訪問和檢索非關(guān)鍵記錄字段的數(shù)據(jù)，即 DBMS 能夠?qū)讉€大文件夾中邏輯相關(guān)的數(shù)據(jù)組織并連接在一起。確定這些邏輯關(guān)系是數(shù)據(jù)管理者的任務(wù)，由數(shù)據(jù)定義語言完成。在該邏輯方式中，記錄通過指針鏈接在一起。在記賬期間，顧 客可在不同時間購買許多東西。顧客文件中的每個記錄都包含這樣一個字段，該字段指向發(fā)票文件中該顧客的第一個發(fā)票的記錄位置，該發(fā)票記錄又依次與該顧客的下一個發(fā)票記錄相連，此鏈接的最后一個發(fā)票記錄由一個作為指針的特殊字符標(biāo)識。該邏輯方式中，數(shù)據(jù)單元的多級結(jié)構(gòu)類似一棵“倒立”的樹，該樹的樹根在頂部，而樹枝向下延伸。在樹型結(jié)構(gòu)中，樹枝不能相連。網(wǎng)狀結(jié)構(gòu)不像樹型結(jié)構(gòu)那樣不允許樹枝相連，它允許節(jié)點間多個方向連接，這樣，每個節(jié)點都可能有幾個所有者，中央電視臺它又可能擁有任意多個其他數(shù)據(jù)單元。 關(guān)系型結(jié)構(gòu)。例如，可建立一些關(guān)系表，將大學(xué)課程同任課教師及上課地點連接起來。這是一個相當(dāng)新穎的數(shù)據(jù)庫組織技術(shù)，將來有望得到廣泛應(yīng)用。人們總是為了各自的目的，按邏輯方式設(shè)想或組織數(shù)據(jù)。記錄在介質(zhì)和硬件中的物理結(jié)構(gòu)不僅取決于所采用的 I/O 設(shè)備、存儲設(shè)備及輸入輸出和存取技術(shù)，而且還取決于用戶定義的 R1 和 R2 中數(shù)據(jù)的邏輯關(guān)系。簡言之，在許多計算機化的信息 記錄中，存儲記錄的物理位置用戶是看不見的
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