數(shù)據(jù)庫管理外文文獻(xiàn)

1、數(shù)據(jù)庫管理數(shù)據(jù)庫（有時拼成database）也稱為電子數(shù)據(jù)庫，是指由計(jì)算機(jī)特別組織的用下快速查找和檢索的任意的數(shù)據(jù)或信息集合。數(shù)據(jù)庫與其它數(shù)據(jù)處理操作協(xié)同工作，其結(jié)構(gòu)要有助于數(shù)據(jù)的存儲、檢索、修改和刪除。數(shù)據(jù)庫可存儲在磁盤或磁帶、光盤或某些輔助存儲設(shè)備上。一個數(shù)據(jù)庫由一個文件或文件集合組成。這些文件中的信息可分解成一個個記錄，每個記錄有一個或多個域。域是數(shù)據(jù)庫存儲的基本單位，每個域一般含有由數(shù)據(jù)庫描述的屬于實(shí)體的一個方面或一個特性的信息。用戶使用鍵盤和各種排序命令，能夠快速查找、重排、分組并在查找的許多記錄中選擇相應(yīng)的域，建立特定集上的報表。數(shù)據(jù)庫記錄和文件的組織必須確保能對信息進(jìn)行檢索。早期

2、的系統(tǒng)是順序組織的（如:字母順序、數(shù)字順序或時間順序）；直接訪問存儲設(shè)備的研制成功使得通過索引隨機(jī)訪問數(shù)據(jù)成為可能。用戶檢索數(shù)據(jù)庫信息的主要方法是query（查詢）。通常情況下，用戶提供一個字符串，計(jì)算機(jī)在數(shù)據(jù)庫中尋找相應(yīng)的字符序列，并且給出字符串在何處出現(xiàn)。比如，用戶必須能在任意給定時間快速處理內(nèi)部數(shù)據(jù)。而且，大型企業(yè)和其它組織傾向于建立許多獨(dú)立的文件，其中包含相互關(guān)聯(lián)的甚至重疊的數(shù)據(jù)，這些數(shù)據(jù)、處理活動經(jīng)常需要和其它文件的數(shù)據(jù)相連。為滿足這些要求，開發(fā)郵各種不同類型的數(shù)據(jù)庫管理系統(tǒng)，如：非結(jié)構(gòu)化的數(shù)據(jù)庫、層次型數(shù)據(jù)庫、網(wǎng)絡(luò)型數(shù)據(jù)庫、關(guān)系型數(shù)據(jù)庫、面向?qū)ο笮蛿?shù)據(jù)庫。在非結(jié)構(gòu)化的數(shù)據(jù)庫中，按照

3、實(shí)體的一個簡單列表組織記錄；很多個人計(jì)算機(jī)的簡易數(shù)據(jù)庫是非結(jié)構(gòu)的。層次型數(shù)據(jù)庫按樹型組織記錄，每一層的記錄分解成更小的屬性集。層次型數(shù)據(jù)庫在不同層的記錄集之間提供一個單一鏈接。與此不同，網(wǎng)絡(luò)型數(shù)據(jù)庫在不同記錄集之間提供多個鏈接，這是通過設(shè)置指向其它記錄集的鏈或指針來實(shí)現(xiàn)的。網(wǎng)絡(luò)型數(shù)據(jù)庫的速度及多樣性使其在企業(yè)中得到廣泛應(yīng)用。當(dāng)文件或記錄間的關(guān)系不能用鏈表達(dá)時，使用關(guān)系型數(shù)據(jù)庫。一個表或一個“關(guān)系”，就是一個簡單的非結(jié)構(gòu)列表。多個關(guān)系可通過數(shù)學(xué)關(guān)系提供所需信息。面向?qū)ο蟮臄?shù)據(jù)庫存儲并處理更復(fù)雜的稱為對象的數(shù)據(jù)結(jié)構(gòu)，可組織成有層次的類，其中的每個類可以繼承層次鏈中更高一級類的特性，這種數(shù)據(jù)庫結(jié)構(gòu)最

4、靈活，最具適應(yīng)性。很多數(shù)據(jù)庫包含自然語言文本信息，可由個人在家中使用。小型及稍大的數(shù)據(jù)庫在商業(yè)領(lǐng)域中占有越來越重要的地位。典型的商業(yè)應(yīng)用包括航班預(yù)訂、產(chǎn)品管理、醫(yī)院的醫(yī)療記錄以及保險公司的合法記錄。最大型的數(shù)據(jù)庫通常用天政府部門、企業(yè)、大專院校等。這些數(shù)據(jù)庫存有諸如摘要、報表、成文的法規(guī)、通訊錄、報紙、雜志、百科全書、各式目錄等資料。索引數(shù)據(jù)庫包含參考書目或用于找到相關(guān)書籍、期刊及其它參考文獻(xiàn)的索引。目前有上萬種可公開訪問的數(shù)據(jù)庫，內(nèi)容包羅萬象，從法律、醫(yī)學(xué)、工程到新聞、時事、游戲、分類廣告、指南等?？茖W(xué)家、醫(yī)生、律師、財(cái)經(jīng)分析師、股票經(jīng)紀(jì)人等專家和各類研究者越來越多地依賴這些數(shù)據(jù)庫從大量的信

5、息中做快速的查找訪問。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(xiàng)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。邏輯結(jié)構(gòu)。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。dbms在存儲、訪問和檢索操作過程中可選用以下邏輯構(gòu)造技術(shù)：鏈表結(jié)構(gòu)。在該邏輯方式中，記錄通過指針鏈接在一起。指針是記錄集中的一個數(shù)據(jù)項(xiàng)，它指出另一個邏輯相關(guān)的記錄的存儲位置，例如，顧客主文件中的記錄將包含每個顧客的姓名和地址，而且該文件中的每個

6、記錄都由一個賬號標(biāo)識。在記賬期間，顧客可在不同時間購買許多東西。公司保存一個發(fā)票文件以反映這下地交易，這種情況下可使用鏈表結(jié)構(gòu)，以顯示給定時間內(nèi)未支付的發(fā)票。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。層次（樹型）結(jié)構(gòu)。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。在層次（樹型）結(jié)構(gòu)中存在主-從關(guān)系，惟一的根數(shù)據(jù)下是從屬的元或節(jié)點(diǎn)，而每個元或樹枝都只有一個所有者，這樣，一個customer（顧客）擁有一個in

7、voice（發(fā)票），而invoice（發(fā)票）又有從屬項(xiàng)。在樹型結(jié)構(gòu)中，樹枝不能相連。網(wǎng)狀結(jié)構(gòu)。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(diǎn)間多個方向連接，這樣，每個節(jié)點(diǎn)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。數(shù)據(jù)管理軟件允許從文件的任一記錄開始提取該結(jié)構(gòu)中的所需信息。關(guān)系型結(jié)構(gòu)。關(guān)系型結(jié)構(gòu)由許多表格組成，數(shù)據(jù)則以“關(guān)系”的形式存儲在這些表中。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(diǎn)連接起來。為了找到英語課的上課地點(diǎn)和教師名，首先查詢課程/教師關(guān)系表得到名字（為“fitt”），再查詢課程/地點(diǎn)關(guān)系表得到地點(diǎn)（“main 142”），當(dāng)然，也可能有其他關(guān)

8、系。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。物理結(jié)構(gòu)。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)。因此，在一個具體應(yīng)用中，記錄r1和r2是邏輯相連且順序處理的，但是，在計(jì)算機(jī)系統(tǒng)中，這些在一個應(yīng)用中邏輯相鄰的記錄，物理位置完全可能不在一起。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。例如，r1和r2可能是持有信用卡的顧客記錄，而顧客要求每兩周將貨物運(yùn)送到同一個城市的同一個街區(qū)，而從運(yùn)輸部門的管理者看，r1和r2是按地理位置組織的運(yùn)輸記錄的順序項(xiàng)，但是在a/r應(yīng)用中，可找到r1長表

9、示的顧客，并且可根據(jù)其完全不同的賬號處理他們的賬目。簡言之，在許多計(jì)算機(jī)化的信息記錄中，存儲記錄的物理位置用戶是看不見的。oracle的數(shù)據(jù)庫管理功能oracle 包括許多使數(shù)據(jù)庫易于管理的功能，分三部分討論：oracle 企業(yè)管理器、附加包、備份和恢復(fù)。1. oracle 企業(yè)管理器和任何數(shù)據(jù)庫服務(wù)器一樣，oracle 數(shù)據(jù)庫服務(wù)器包括以下部分：oracle 企業(yè)管理器（im）、一個帶有圖形接口的用于管理數(shù)據(jù)庫用戶、實(shí)例和提供oracle 環(huán)境等附加信息功能（如：復(fù)制）的數(shù)據(jù)庫管理工具框架。在oracle8i數(shù)據(jù)庫之前，em 軟件必須安裝在windows95/98或者基于nt 的系統(tǒng)中，而且

10、每個庫每次只能由一個數(shù)據(jù)庫管理者訪問。如今你可以通過瀏覽器或者把em 裝入window95/98/2000 或基于nt 的系統(tǒng)中來使用em。多個數(shù)據(jù)庫管理員可以同時訪問em庫。在oracle9i的em版中，超級管理員可以定義在普通管理員的控制臺上顯示的服務(wù)，并能建立管理區(qū)域。2. 附加包正如下面所描述的那樣，oracle可使用一些可選的附加包，還有用于oracle應(yīng)用程序和sap r/3的管理包。（1）標(biāo)準(zhǔn)管理包oracle的標(biāo)準(zhǔn)管理包提供了用于小型oracle數(shù)據(jù)庫的管理工具（如：oracle服務(wù)器/標(biāo)準(zhǔn)版）。功能包括：對數(shù)據(jù)庫爭用、輸入/輸出、裝載、內(nèi)存使用和實(shí)例、對話分析、索引調(diào)整進(jìn)行監(jiān)

11、控，并改變調(diào)查和跟蹤。（2）診斷包利用診斷包，可以監(jiān)控、診斷及維護(hù)企業(yè)版數(shù)據(jù)庫、操作系統(tǒng)和應(yīng)用程序的安全。用有關(guān)歷史和實(shí)時的分析，可自動的在問題發(fā)生前將其消除。診斷包還提供空間管理功能，有助于對未來系統(tǒng)資源需要的計(jì)劃和跟蹤。（3）調(diào)整包利用調(diào)整包，可確定并調(diào)整企業(yè)版數(shù)據(jù)庫和應(yīng)用系統(tǒng)的瓶頸，如效率低的sql、很差的數(shù)據(jù)設(shè)計(jì)、系統(tǒng)資源的不當(dāng)使用，從而優(yōu)化系統(tǒng)性能。調(diào)整包能提前發(fā)現(xiàn)調(diào)整時機(jī)，并自動生成分析和需求變化來調(diào)整系統(tǒng)。（4）變化管理包變化管理包在升級企業(yè)版數(shù)據(jù)庫時幫助排錯和避免丟失數(shù)據(jù)，以達(dá)到支持新的應(yīng)用程序的目的。該包能分析與應(yīng)用程序變動有關(guān)的影響和復(fù)雜依賴關(guān)系并自動升級數(shù)據(jù)庫。用戶可使用

12、一種簡單的向?qū)О幢匾牟襟E來升級。（5）可用性oracle 企業(yè)管理器可用管理oracle標(biāo)準(zhǔn)版或企業(yè)版。在標(biāo)準(zhǔn)版中，用于診斷、調(diào)整和改變實(shí)例的附加功能由標(biāo)準(zhǔn)管理包提供。對于企業(yè)版，這些附加的功能由單獨(dú)的診斷包、調(diào)整包和變化管理包提供。3. 備份和恢復(fù)正如每個數(shù)據(jù)庫管理者所熟知的，對數(shù)據(jù)庫做備份是一件很普通但又必要的工作。一次不當(dāng)?shù)膫浞輹箶?shù)據(jù)庫難于恢復(fù)甚至不可恢復(fù)。不幸的是，人們往往在相關(guān)系統(tǒng)發(fā)生故障而丟失了重要的業(yè)務(wù)數(shù)據(jù)后才認(rèn)識到這項(xiàng)日常工作的重要。下面介紹一些實(shí)現(xiàn)數(shù)據(jù)庫備份操作的產(chǎn)品技術(shù)。 (1)恢復(fù)管理者典型的備份包括完整的數(shù)據(jù)庫備份（最普通的類型）、桌面空間備份、數(shù)據(jù)文件備份、控件備

13、份和存檔注冊備份。oracle8i為數(shù)據(jù)服務(wù)器管理備份和恢復(fù)管理器（rman）。以前，oracle的企業(yè)備份工具（ebu）在一些平臺上提供了相似的解決方案。然而，rman及其存儲在oracle數(shù)據(jù)庫中的恢復(fù)目錄提供了更完整的解決方案。rman可以自動定位、備份、存儲并恢復(fù)數(shù)據(jù)文件、控制文件和存檔記錄注冊。當(dāng)備份到期時，oracle9i的rman可以重新啟動備份和恢復(fù)來實(shí)現(xiàn)恢復(fù)窗口的任務(wù)。oracle企業(yè)管理器的備份管理器曾rman提供基于圖形用戶界面的接口。(2)附加備份和恢復(fù)rman能夠執(zhí)行企業(yè)版數(shù)據(jù)庫的附加備份。附加備份僅備份上一次備份后改變了的數(shù)據(jù)文件、桌面空間或數(shù)據(jù)庫塊，因此，它比完整

14、的備份占用時間短而且速度快。rman也能執(zhí)行及時指向的恢復(fù)，這種恢復(fù)能在一個不期望的事件發(fā)片之前（如錯誤的刪除表格）恢復(fù)數(shù)據(jù)。(3)連續(xù)存儲管理器許多媒體軟件商支持rman。oracle捆綁了連續(xù)存儲管理器來提供媒體管理服務(wù)，包括為至多四臺設(shè)備提供磁帶容量跟蹤的服務(wù)。rman界面自動地與媒體管理軟件一起來管理備份和恢復(fù)操作必須的磁帶設(shè)備。(4)可用性盡管標(biāo)準(zhǔn)版和企業(yè)版的oracle都有基本的恢復(fù)機(jī)制，但附加備份僅限于企業(yè)版。oracle 和 sql server 的比較選擇我不得不決定是使用oracle數(shù)據(jù)庫及其數(shù)據(jù)庫開發(fā)系統(tǒng)，還是選擇配有visual studio的microsoft sql

15、 server。這個決策將決定我們今后web項(xiàng)目的方向。這兩種組合各有什么優(yōu)勢和劣勢呢？lori: 決定選擇哪種方案將取決于你目前的工作平臺。例如，如果你想實(shí)現(xiàn)一種基于web的數(shù)據(jù)庫應(yīng)用，而且你的工作平臺只是windows,那么sql sever和visual studio 組件就是一個不錯的選擇。但是對于混合平臺，則最好選擇oracle解決方案。還要考慮一些其他的因素，例如你可以獲得哪些額外的功能以及需要哪些技術(shù)。webdb是一種內(nèi)容管理和開發(fā)工具。webdb是一種基于瀏覽器的工具，有利于方便地創(chuàng)建內(nèi)容，以及提供導(dǎo)航和維護(hù)工具。這對于已經(jīng)使用oracle的機(jī)構(gòu)則是一個很好的解決方案。orac

16、le比sql server更易于調(diào)整，但你需要身邊有一個稱職的oracle管理員。sql server/visual studio方法相對來說較難使用，需要一個有經(jīng)驗(yàn)的面向?qū)ο蟮木幊倘藛T或一些全面的訓(xùn)練。但是，你只要花上1619美元就可以獲得許多visual studio的開發(fā)工具：visual basic，visual c+，以及visual interdev。另外，你需要再加上sql server的費(fèi)用，1999美元就可以擁有10個客戶端，或用3999美元獲得25個客戶端-與oracle的費(fèi)用相比則便宜些。oracle也有一個組件解決方案，根據(jù)所選擇平臺的不同，費(fèi)用也不同，最低為6767美

17、元。o套件不僅包括webdb和oracle8，還包括其他的開發(fā)工具，如oracle應(yīng)用服務(wù)器、jdeveloper和iworkplace模板，該套件與microsoft 方案相比能運(yùn)行在更多的平臺上。如果你剛創(chuàng)業(yè)或是一個小型或中型企業(yè)，則oracle套件是一個很好的選擇方案。以組件的形式購買這些工具要比單獨(dú)采購的花費(fèi)要少些。對這兩種方案的選擇主要取決于你的技術(shù)水平、硬件資源和資金預(yù)算。我希望以上所說的能對你的決策有一定幫助。brooks: 我完全同意對這種方案的選擇在很大程度上取決于你目前所配有的基礎(chǔ)設(shè)施和所具有的專門技術(shù)。如果很難做出抉擇，你就需要考慮由誰來做這項(xiàng)工作，以及

18、你的重點(diǎn)是什么。這兩種產(chǎn)品采用不同的方法，并反映出這兩個開發(fā)商的不同特點(diǎn)。oracle是為那些優(yōu)秀的程序員和項(xiàng)目負(fù)責(zé)人進(jìn)行的專業(yè)化開發(fā)而設(shè)計(jì)的產(chǎn)品。學(xué)習(xí)時間校長，該方案的費(fèi)用也較高；但是如果你堅(jiān)持下去，最終就會獲得更好的靈活性，以及更高的可靠性。如果你的項(xiàng)目時間很緊，而且你沒有時間或金錢來雇傭一個花銷很大、經(jīng)驗(yàn)非常豐富的開發(fā)隊(duì)伍，你也許會發(fā)現(xiàn)選擇oracle方案會很容易使你陷入困境。沒有比開發(fā)一個質(zhì)量很差的oracle應(yīng)用程序更糟糕的了。microsoft 所提供的解決方案則致力于快速開發(fā)和低本實(shí)現(xiàn)。開發(fā)工具、服務(wù)器、以及你所需要的開發(fā)人員的花費(fèi)都比較少。若要使項(xiàng)目快速啟動，選擇sql ser

19、ver和visual studio則是很好的方式。當(dāng)然，有得必有失。在使用visual studio 和sql server 的過程中，我所遇到的主要問題就是，只能使用microsoft 操作系統(tǒng)和intel硬件。如果有一天你需要支持成百上千個用戶，你除了購買上百個服務(wù)器而外別無他法，這會給管理帶來很大麻煩。如果你采用microsoft 方法，就表明你可能只需要visual interdev即可。如果你已經(jīng)知道你將開發(fā)visual basic或visual c+里的activex組件這就是一個預(yù)示，表明你需要更多的來考慮oracle的解決方案。我想強(qiáng)調(diào)的是，盡管這兩種平臺有它們各自的優(yōu)點(diǎn)和缺點(diǎn)

20、，如果你使用正確，你用任何一個都可以設(shè)計(jì)出具有世界水平的應(yīng)用程序。如果你的組織傾向于某種產(chǎn)品，一定要跟隨這種趨向。如果你是從零起點(diǎn)開始，就需要問問自己，你的組織是更傾向于完美主義還是實(shí)用主義，并要清楚這兩種主義都有缺點(diǎn)。database management database (sometimes spelled database) is also called an electronic database, referring to any collections of data, or information, that is specially organized for rapid s

21、earch and retrieval by a computer. databases are structured to facilitate the storage, retrieval modification and deletion of data in conjunction with various data-processing operations. database can be stored on magnetic disk or tape, optical disk, or some other secondary storage device.a database

22、consists of a file or a set of files. the information in the these files may be broken down into records, each of which consists of one or more fields are the basic units of data storage, and each field typically contains information pertaining to one aspect or attribute of the entity described by t

23、he database. using keywords and various sorting commands, users can rapidly search, rearrange, group, and select the fields in many records to retrieve or create reports on particular aggregates of data.database records and files must be organized to allow retrieval of the information. early system

24、were arranged sequentially (i.e., alphabetically, numerically, or chronologically); the development of direct-access storage devices made possible random access to data via indexes. queries are the main way users retrieve database information. typically the user provides a string of characters, and

25、the computer searches the database for a corresponding sequence and provides the source materials in which those characters appear. a user can request, for example, all records in which the content of the field for a persons last name is the word smith.the many users of a large database must be able

26、 to manipulate the information within it quickly at any given time. moreover, large business and other organizations tend to build up many independent files containing related and even overlapping data, and their data, processing activities often require the linking of data from several files. sever

27、al different types of database management systems have been developed to support these requirements: flat, hierarchical, network, relational, and object-oriented. in flat databases, records are organized according to a simple list of entities; many simple databases for personal computers are flat in

28、 structure. the records in hierarchical databases are organized in a treelike structure, with each level of records branching off into a set of smaller categories. unlike hierarchical databases, which provide single links between sets of records at different levels, network databases create multiple

29、 linkages between sets by placing links, or pointers, to one set of records in another; the speed and versatility of network databases have led to their wide use in business. relational databases are used where associations among files or records cannot be expressed by links; a simple flat list beco

30、mes one table, or “relation”, and multiple relations can be mathematically associated to yield desired information. object-oriented databases store and manipulate more complex data structures, called “objects”, which are organized into hierarchical classes that may inherit properties from classes hi

31、gher in the chain; this database structure is the most flexible and adaptable.the information in many databases consists of natural-language texts of documents; number-oriented database primarily contain information such as statistics, tables, financial data, and raw scientific and technical data. s

32、mall databases can be maintained on personal-computer systems and may be used by individuals at home. these and larger databases have become increasingly important in business life. typical commercial applications include airline reservations, production management, medical records in hospitals, and

33、 legal records of insurance companies. the largest databases are usually maintained by governmental agencies, business organizations, and universities. these databases may contain texts of such materials as catalogs of various kinds. reference databases contain bibliographies or indexes that serve a

34、s guides to the location of information in books, periodicals, and other published literature. thousands of these publicly accessible databases now exist, covering topics ranging from law, medicine, and engineering to news and current events, games, classified advertisements, and instructional cours

35、es. professionals such as scientists, doctors, lawyers, financial analysts, stockbrokers, and researchers of all types increasingly rely on these databases for quick, selective access to large volumes of information.dbms structuring techniquessequential, direct, and other file processing approaches

36、are used to organize and structure data in single files. but a dbms is able to integrate data elements from several files to answer specific user inquiries for information. that is, the dbms is able to structure and tie together the logically related data from several large files. logical structures

37、. identifying these logical relationships is a job of the data administrator. a data definition language is used for this purpose. the dbms may then employ one of the following logical structuring techniques during storage access, and retrieval operations.list structures. in this logical approach, r

38、ecords are linked together by the use of pointers. a pointer is a data item in one record that identifies the storage location of another logically related record. records in a customer master file, for example, will contain the name and address of each customer, and each record in this file is iden

39、tified by an account number. during an accounting period, a customer may buy a number of items on different days. thus, the company may maintain an invoice file to reflect these transactions. a list structure could be used in this situation to show the unpaid invoices at any given time. each record

40、in the customer in the invoice file. this invoice record, in turn, would be linked to later invoices for the customer. the last invoice in the chain would be identified by the use of a special character as a pointer.hierarchical (tree) structures. in this logical approach, data units are structured

41、in multiple levels that graphically resemble an “upside down” tree with the root at the top and the branches formed below. theres a superior-subordinate relationship in a hierarchical (tree) structure. below the single-root data component are subordinate elements or nodes, each of which, in turn, “o

42、wn” one or more other elements (or none). each element or branch in this structure below the root has only a single owner. thus, a customer owns an invoice, and the invoice has subordinate items. the branches in a tree structure are not connected.network structures. unlike the tree approach, which d

43、oes not permit the connection of branches, the network structure permits the connection of the nodes in a multidirectional manner. thus, each node may have several owners and may, in turn, own any number of other data units. data management software permits the extraction of the needed information f

44、rom such a structure by beginning with any record in a file.relational structures. a relational structure is made 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 c

45、ourse, 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 (“fitt”), and the course/location relation is a relatively new database structuring approach thats expected to be widely impl

46、emented 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 computer system its quite possible that these records th

47、at 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

48、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 managers perspective, then, r1 and r2 are sequential entries on a geographically organized shipping report. but in

49、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 computer-based information systems is invisible to us

50、ers. database management features of oracle oracle includes many features that make the database easier to manage. weve divided the discussion in this section into three categories: oracle enterprise manager, add-on packs, backup and recovery.1.oracle enterprise manageras part of every database serv

51、er, oracle provides the oracle enterprise manager (em), a database management tool framework with a graphical interface used to manage database users, instances, and features (such as replication) that can provide additional information about the oracle environment.prior to the oracle8i database, th

52、e em software had to be installed on windows 95/98 or nt-based systems and each repository could be accessed by only a single database manager at a time. now you can use em from a browser or load it onto windows 95/98/2000 or nt-based systems. multiple database administrators can access the em repos

53、itory at the same time. in the em repository for oracle9i, the super administrator can define services that should be displayed on other administrators consoles, and management regions can be set up.2.add-on packsseveral optional add-on packs are available for oracle, as described in the following s

54、ections. in addition to these database-management packs, management packs are available for oracle applications and for sap r/3.(1)standard management pack the standard management pack for oracle provides tools for the management of small oracle databases (e.g., oracle server/standard edition). feat

55、ures include support for performance monitoring of database contention, i/o, load, memory use and instance metrics, session analysis, index tuning, and change investigation and tracking.(2)diagnostics pack you can use the diagnostic pack to monitor, diagnose, and maintain the health of enterprise ed

56、ition databases, operating systems, and applications. with both historical and real-time analysis, you can automatically avoid problems before they occur. the pack also provides capacity planning features that help you plan and track future system-resource requirements. (3)tuning packwith the tuning

57、 pack, you can optimise system performance by identifying and tuning enterprise edition databases and application bottlenecks such as inefficient sql, poor data design, and the improper use of system resources. the pack can proactively discover tuning opportunities and automatically generate the ana

58、lysis and required changes to tune the systems. (4)change management packthe change management pack helps eliminate errors and loss of data when upgrading enterprise edition databases to support new applications. it impact and complex dependencies associated with application changes and automaticall

59、y perform database upgrades. users can initiate changes with easy-to-use wizards that teach the systematic steps necessary to upgrade.(5)availabilityoracle enterprise manager can be used for managing oracle standard edition and/or enterprise edition. additional functionality is provided by separate diagnostics, tuning, and chang