【正文】 involves the support of anull value,which is distinct from an empty string or anumber with avalue of course,this can39。t applytoprimary addition,mostdatabase implementations support the conceptof anunnull fieldconstraint that preventsnullvaluesin ，空值的系統(tǒng)化處理 RDBMS 支持空值 (不同于空的字符串或空白字符串，并且不為 0)系統(tǒng)化的表示缺少的信息，且與數(shù)據(jù)類型無關(guān)。 Rule4： DynamicOnLine Catalog Basedon the Relational Model Arelationaldatabase must provide accessto its structure through the same tools that are usedto access the usually acplishedby storing the structure definition within special system ，基于關(guān)系模型的聯(lián)機目錄數(shù)據(jù)庫的 描述在邏輯上應該和一般數(shù)據(jù)采用同樣的方式，使得授權(quán)用戶可以使用查詢一般數(shù)據(jù)所用的關(guān)系語言來查詢數(shù)據(jù)庫的描述信息。 Rule5： Comprehensive Data SublanguageRule Thedatabase must support at least one clearly defined language that includes functionalityfor data definition,data manipulation,data integrity,anddatabasetransaction mercial relational databasesuse formsof the standard SQL(Structured Query Language)as their supported prehensive ，合理廣泛的子語言準則一個關(guān)系系統(tǒng)可以具有幾種語言和多種終端使用方式 (表格填空方式，命令方式等 )。但是必須有一種語言，它的語句可以表示為具有嚴格語法規(guī)定的字符串，并能全面的支持以下功能：數(shù)據(jù)定義，視圖定義，數(shù)據(jù)操作 ，完整約束，授權(quán)，事物控制。 Rule6： View UpdatingRule Data can be presentedto theuserin different logical binations,called should support the samefull rangeof data manipulation that directaccessto atable has practice,providingupdateanddelete accessto logical viewsis difficultandisnot fully ，視圖更新準則所有理論上可更新的視圖也應該允許由系統(tǒng)更新。 Rule7： HighlevelInsert,Update,andDelete Data can be retrievedfrom arelationaldatabasein sets constructedof datafrom multiple rowsand/or multiple states thatinsert,update,anddelete operations should be supportedforany retrievableset rather than justfor asingle rowin ，高階的插入，更新和刪除把一個基本關(guān)系或?qū)С鲫P(guān)系作為一個操作對象進行數(shù)據(jù)的檢索以及插入，更新和刪除。 Rule8： Physical Data Independence Theuseris isolatedfrom the physical methodof storingand retrieving informationfrom can be madeto the underlying architecture(hardware,disk storage methods)without affecting how theuser accesses ，數(shù)據(jù)的物理獨立性無論數(shù)據(jù)庫的數(shù)據(jù)在存儲表示上或存取方法上做任何變化，應用程序和終端活動要都保持邏輯上的不變性。 Rule9： Logical Data Independence How auser views data shouldnot changewhen the logical structure(tables structure)of thedatabase particularly difficultto databases relyon strong tiesbetween theuserviewof the dataand the actual structureof the underlying ，數(shù)據(jù)的邏輯獨立性當基本表中進行理論上信息不受損害的任何變化時，應用程序和終端和終端活動都要保持邏輯上的不變性。 Rule10： Integrity Independence Thedatabase language(like SQL)should support constraintsonuser input that maintaindatabase fully implementedby most major aminimum,all databases do preserve two constraints through ponentof aprimarykey can have anull value.(seerule3)If aforeignkeyis definedin oable,any valuein it must existas aprimarykeyin ，數(shù)據(jù)完整的獨立性關(guān)系數(shù)據(jù)庫的完整性 約束必須是用數(shù)據(jù)子語言定義并存貯在目錄中的，而不是在應用程序中加以定義的。至少要支持以下兩種約束：實體完整性：主鍵中的屬性不允許為 NULL；參照完整性：對于關(guān)系數(shù)據(jù)庫中每個不同的非空的外碼值，必須存在一個取自同一個域匹配的主鍵值。 Rule11： Distribution Independence Auser should be totally unawareof whetherornot thedatabaseisdistributed(whether partsof thedatabase existin multiple locations).A varietyof reasons make thisrule difficultto implement； I will spend time addressing these reasonswhen we discussdistributed ，分布的獨立性一個RDBMS 應該具有分布獨立性。用戶不必了解數(shù)據(jù)庫是否是分布式的。 (無論數(shù)據(jù)庫是否有部分處于復雜多重環(huán)境中 ) Rule12： NonsubversionRule There should be no wayto modify thedatabase structure other than through the multiple rowdatabase language(like SQL).12，無破壞準則若 RDBMS 有某種低級語言，這一低級語言不能違背或繞過完整性準則以及高級關(guān)系語言表達的約束。 Most databases today support administrative tools that allowsome direct manipulationof the the lifeof this newsletter,I will be expandingon the concepts coveredby eachof Codd39。s will use the relational query language of choice,SQL,to illustrate these concepts and explain relational database structure in detail. 特別聲明： 1：資料來源 于互聯(lián)網(wǎng)，版權(quán)歸屬原作者 2：資料內(nèi)容屬于網(wǎng)絡意見，與本賬號立場無關(guān) 3：如有侵權(quán)，請告知，立即刪除。