翻译文献-看一个实际操作的例子

外文原文NOWLETSLOOKATANEXAMPLETHATACTUALLYDOESSOMETHINGOPERATIONALLYCONSIDERTHEFOLLOWINGSELECTNAME,TYPE_IDFROMFOODSTHISPERFORMSAPROJECTIONONFOODS,SELECTINGTWOOFITSTHREECOLUMNSNAMEANDTYPE_IDTHEIDCOLUMNISTHROWNOUT,ASSHOWNINFIGURE45FIGURE45APROJECTIONOFFOODSLETSSUMMARIZETHEFROMCLAUSETAKESTHEINPUTRELATIONSANDPERFORMSAJOIN,WHICHCOMBINESTHEMINTOASINGLERELATIONR1THEWHERECLAUSETAKESR1ANDFILTERSITVIARESTRICTION,PRODUCINGANEWRELATIONR2THESELECTCLAUSETAKESR2ANDPERFORMSPROJECTION,PRODUCINGTHEFINALRESULTTHISPROCESSISSHOWNINFIGURE46FIGURE46RESTRICTIONANDPROJECTIONINSELECTWITHTHISSIMPLEEXAMPLE,YOUCANBEGINTOSEEHOWAQUERYLANGUAGEINGENERALANDSQLINPARTICULARULTIMATELYOPERATESINTERMSOFRELATIONALOPERATIONSTHEREISREALMATHUNDERTHEHOODFILTERINGIFTHESELECTCOMMANDISTHEMOSTCOMPLEXCOMMANDINSQL,THENTHEWHERECLAUSEISTHEMOSTCOMPLEXCLAUSEINSELECTAND,JUSTASTHESELECTCOMMANDPULLSINASPECTSOFSETTHEORY,THEWHERECLAUSEALSOPULLSINASPECTSOFFORMALLOGICBYANDLARGE,THEWHERECLAUSEISUSUALLYTHEPARTOFTHESELECTCOMMANDTHATHARBORSTHEMOSTCOMPLEXITYBUTITALSODOESMOSTOFTHEWORKHAVINGASOLIDUNDERSTANDINGOFITSMECHANICSWILLMOSTLIKELYBRINGTHEBESTOVERALLRETURNSINYOURDAYTODAYUSEOFSQLTHEDATABASEAPPLIESTHEWHERECLAUSETOEACHROWOFTHERELATIONPRODUCEDBYTHEFROMCLAUSER1ASSTATEDEARLIER,WHEREARESTRICTIONISAFILTERTHEARGUMENTOFWHEREISALOGICALPREDICATEAPREDICATE,INTHESIMPLESTSENSE,ISJUSTANASSERTIONABOUTSOMETHINGCONSIDERTHEFOLLOWINGSTATEMENTTHEDOGSUBJECTISPURPLEANDHASATOOTHYGRINPREDICATETHEDOGISTHESUBJECTANDTHEPREDICATECONSISTSOFTHETWOASSERTIONSCOLORISPURPLEANDGRINISTOOTHYTHISSTATEMENTMAYBETRUEORFALSE,DEPENDINGONTHEDOGTHEPREDICATEISAPPLIEDTOINTHETERMINOLOGYOFFORMALLOGIC,ASTATEMENTCONSISTINGOFASUBJECTANDAPREDICATEISCALLEDAPROPOSITIONALLPROPOSITIONSAREEITHERTRUEORFALSEAPREDICATETHENSAYSSOMETHINGABOUTASUBJECTTHESUBJECTINTHEWHERECLAUSEISAROWTHEROWISTHELOGICALSUBJECTTHEWHERECLAUSEISTHELOGICALPREDICATETOGETHERASINAGRAMMATICALSENTENCETHEYFORMALOGICALPROPOSITION,WHICHEVALUATESTOTRUEORFALSETHISPROPOSITIONISFORMULATEDANDEVALUATEDFOREVERYROWINR1EACHROWINWHICHTHEPROPOSITIONEVALUATESTOTRUEISINCLUDEDORSELECTEDASPARTOFTHERESULTR2EACHROWINWHICHITISFALSEISEXCLUDEDSOTHEDOGPROPOSITIONTRANSLATEDINTOARELATIONALEQUIVALENTWOULDLOOKSOMETHINGLIKETHISSELECTFROMDOGSWHERECOLORPURPLEANDGRINTOOTHYTHEDATABASEWILLTAKEEACHROWINRELATIONDOGSTHESUBJECTANDAPPLYTHEWHERECLAUSETHEPREDICATETOFORMTHELOGICALPROPOSITIONTHISROWHASCOLORPURPLEANDGRINTOOTHYTHISISEITHERTRUEOFTHEGIVENROW,ORITISFALSENOTHINGMOREIFITISTRUE,THENTHEROWORDOGISINDEEDPURPLEANDTOOTHY,ANDITISINCLUDEDINTHERESULTIFTHEPROPOSITIONISFALSE,THENTHEROWMAYBEPURPLEBUTNOTTOOTHY,ORTOOTHYBUTNOTPURPLE,ORNEITHERPURPLENORTOOTHYINANYCASE,THEPROPOSITIONISFALSEANDTHEROWISTHEREFOREEXCLUDEDWHEREISAPOWERFULFILTERITPROVIDESYOUWITHAGREATDEGREEOFCONTROLOVERTHECONDITIONSWITHWHICHTOINCLUDEOREXCLUDEROWSINORFROMTHERESULTASALOGICALPREDICATE,ITISALSOALOGICALEXPRESSIONALOGICALEXPRESSIONISANEXPRESSIONTHATEVALUATESTOEXACTLYONEOFTWOPOSSIBLELOGICALOUTCOMESTRUEORFALSEALOGICALPREDICATETHENISJUSTALOGICALEXPRESSIONTHATISUSEDINASPECIFICWAYTOQUALIFYASUBJECTANDFORMAPROPOSITIONATTHEIRSIMPLEST,LOGICALEXPRESSIONSCONSISTOFTWOVALUESORVALUEEXPRESSIONSCOMPAREDBYRELATIONALOPERATORSVALUEEXPRESSIONSAREBUILTFROMVALUESANDOPERATORSARELATIONALOPERATORREFERREDTOHEREISARELATIONALOPERATORINTHEMATHEMATICALSENSEITISDIFFERENTFROMARELATIONALOPERATORINTHERELATIONALSENSE,ASDEFINEDINRELATIONALALGEBRAARELATIONALOPERATORINMATHISANOPERATORTHATRELATESTWOVALUESANDEVALUATESTOTRUEORFALSEEG,XY,XBITWISELEFTSHIFT120SQLITESELECT1SELECT12120SQLITESELECT1AND11AND11LOGICALOPERATORSLOGICALOPERATORSAND,OR,NOT,INAREBINARYOPERATORSTHATOPERATEONTRUTHVALUESORLOGICALEXPRESSIONSTHEYPRODUCEASPECIFICTRUTHVALUEDEPENDINGONTHEIRINPUTSTHEYAREUSEDTOBUILDMORECOMPLEXLOGICALEXPRESSIONSFROMSIMPLEREXPRESSIONS,SUCHASX5ANDX3Y4ANDNOTY0COLORPURPLEANDGRINTOOTHYTHETRUTHVALUEPRODUCEDBYALOGICALOPERATORFORAGIVENPAIROFARGUMENTSDEPENDSONTHEOPERATORFOREXAMPLE,LOGICALANDREQUIRESTHATBOTHINPUTVALUESEVALUATETOTRUEINORDERFORITTORETURNTRUELOGICALOR,ONTHEOTHERHAND,ONLYREQUIRESTHATONEINPUTVALUEEVALUATETOTRUEINORDERFORITTORETURNTRUEALLPOSSIBLEOUTCOMESFORAGIVENLOGICALOPERATORAREDEFINEDINWHATISKNOWNASATRUTHTABLETHETRUTHTABLESFORANDANDORARESHOWNINTABLES43AND44,RESPECTIVELYTABLE43TRUTHTABLEFORLOGICALANDARGUMENT1ARGUMENT2RESULTTRUETRUETRUETRUEFALSEFALSEFALSEFALSEFALSETRUETRUETRUETRUEFALSETRUEFALSEFALSEFALSETHISISTHESTUFFTHEWHERECLAUSEISMADEOFUSINGLOGICALOPERATORS,YOUCANCREATEACOMPLEXLOGICALPREDICATETHEPREDICATEISWHATDEFINESHOWWHERESRELATIONALRESTRICTIONOPERATIONRESTRICTSFOREXAMPLESQLITESELECTFROMFOODSWHERENAMEJUJYFRUITANDTYPE_ID9IDTYPE_IDNAME2449JUJYFRUITTHERESTRICTIONHEREWORKSACCORDINGTOTHEEXPRESSIONNAMEJUJYFRUITANDTYPE_ID9,WHICHCONSISTSOFTWOLOGICALEXPRESSIONSJOINEDBYLOGICALANDBOTHOFTHESECONDITIONSMUSTBETRUEFORANYRECORDINFOODSTOBEINCLUDEDINTHERESULTTHELIKEOPERATORAPARTICULARLYUSEFULRELATIONALOPERATORISLIKELIKEISSIMILARTOEQUALS,BUTISUSEDFORMATCHINGSTRINGVALUESAGAINSTPATTERNSFOREXAMPLE,TOSELECTALLROWSINFOODSWHOSENAMESBEGINWITHTHELETTER“J,”YOUCOULDDOTHEFOLLOWINGSQLITESELECTID,NAMEFROMFOODSWHERENAMELIKEJIDNAME156JUICEBOX236JUICYFRUITGUM243JELLOWITHBANANAS244JUJYFRUIT245JUNIORMINTS370JAMBALAYAAPERCENTSYMBOLINTHEPATTERNMATCHESANYSEQUENCEOFZEROORMORECHARACTERSINTHESTRINGANUNDERSCORE_INTHEPATTERNMATCHESANYSINGLECHARACTERINTHESTRINGTHEPERCENTSYMBOLISGREEDYITWILLEATEVERYTHINGBETWEENTWOCHARACTERSEXCEPTTHOSECHARACTERSIFITISONTHEEXTREMELEFTORRIGHTOFAPATTERN,ITWILLCONSUMEEVERYTHINGONEACHRESPECTIVESIDECONSIDERTHEFOLLOWINGEXAMPLESSQLITESELECTID,NAMEFROMFOODSWHERENAMELIKEACPIDNAME127GUACAMOLEDIP168PEACHSCHNAPPS198MACKINAWPEACHESANOTHERUSEFULTRICKISTOUSENOTTONEGATEAPATTERNSQLITESELECTID,NAMEFROMFOODSWHERENAMELIKEACPANDNAMENOTLIKESCHIDNAME38PIEBLACKBERRYPIE127GUACAMOLEDIP198MACKINAWPEACHESLIMITINGANDORDERINGYOUCANLIMITTHESIZEANDPARTICULARRANGEOFTHERESULTUSINGTHELIMITANDOFFSETKEYWORDSLIMITSPECIFIESTHEMAXIMUMNUMBEROFRECORDSTORETURNOFFSETSPECIFIESTHENUMBEROFRECORDSTOSKIPFOREXAMPLE,THEFOLLOWINGSTATEMENTOBTAINSTHECEREALRECORDTHESECONDRECORDINFOOD_TYPESUSINGLIMITANDOFFSETSELECTFROMFOOD_TYPESLIMIT1OFFSET1ORDERBYIDTHEOFFSETCLAUSESKIPSONEROWTHEBAKERYROWANDTHELIMITCLAUSERETURNSAMAXIMUMOFONEROWTHECEREALROWBUTTHEREISSOMETHINGELSEHEREASWELLORDERBYTHISCLAUSESORTSTHERESULTBYACOLUMNORCOLUMNSBEFOREITISRETURNEDTHEREASONITISIMPORTANTINTHISEXAMPLEISBECAUSETHEROWSRETURNEDFROMSELECTARENEVERGUARANTEEDTOBEINANYSPECIFICORDERTHESQLSTANDARDDECLARESTHISTHUS,THEORDERBYCLAUSEISESSENTIALIFYOUNEEDTOCOUNTONTHERESULTBEINGINANYSPECIFICORDERTHESYNTAXOFTHEORDERBYCLAUSEISSIMILARTOTHESELECTCLAUSEITISACOMMASEPARATEDLISTOFCOLUMNSEACHENTRYMAYBEQUALIFIEDWITHASORTORDERASCASCENDING,THEDEFAULTORDESCDESCENDINGFOREXAMPLESQLITESELECTFROMFOODSWHERENAMELIKEBORDERBYTYPE_IDDESC,NAMELIMIT10IDTYPE_IDNAME38215BAKEDBEANS38315BAKEDPOTATOW/SOUR38415BIGSALAD38515BROCCOLI36214BOUILLABAISSE32812BLT32712BACONCLUBNOTURKE32612BOLOGNA32912BRISKETSANDWICH27410BACONTYPICALLYYOUONLYNEEDTOORDERBYASECONDTHIRD,ETCCOLUMNWHENTHEREAREDUPLICATEVALUESINTHEFIRSTSECOND,ETCORDEREDCOLUMNSHERE,THEREWEREMANYDUPLICATETYPE_IDSIWANTEDTOGROUPTHEMTOGETHER,ANDTHENARRANGETHEFOODSALPHABETICALLYWITHINTHESEGROUPSNOTELIMITANDOFFSETARENOTSTANDARDSQLKEYWORDSASDEFINEDINTHEANSISTANDARDNEVERTHELESS,THEYAREFOUNDONSEVERALOTHERDATABASES,SUCHASMYSQLANDPOSTGRESQLORACLE,MSSQL,ANDFIREBIRDALSOHAVEFUNCTIONALEQUIVALENTS,ALTHOUGHTHEYUSEDIFFERENTSYNTAXIFYOUUSEBOTHLIMITANDOFFSETTOGETHER,YOUCANUSEACOMMANOTATIONINPLACEOFTHEOFFSETKEYWORDFOREXAMPLE,THEFOLLOWINGSQLSELECTFROMFOODSWHERENAMELIKEBORDERBYTYPE_IDDESC,NAMELIMIT1OFFSET2CANBEEXPRESSEDEQUIVALENTLYWITHSQLITESELECTFROMFOODSWHERENAMELIKEBORDERBYTYPE_IDDESC,NAMELIMIT1,2IDTYPE_IDNAME38415BIGSALADHERE,THECOMMAFOLLOWINGLIMIT1ADDSTHEOFFSETOF2TOTHECLAUSEALSO,NOTETHATOFFSETDEPENDSONLIMITTHATIS,YOUCANUSELIMITWITHOUTUSINGOFFSETBUTNOTTHEOTHERWAYAROUNDNOTICETHATLIMITANDOFFSETAREDEADLASTINTHEOPERATIONALPIPELINEONECOMMONMISCONCEPTIONOFLIMIT/OFFSETISTHATITSPEEDSUPAQUERYBYLIMITINGTHENUMBEROFROWSTHATMUSTBECOLLECTEDBYTHEWHERECLAUSETHISISNOTTRUEIFITWERE,THENORDERBYWOULDNOTWORKPROPERLYFORORDERBYTODOITSJOB,ITMUSTHAVETHEENTIRERESULTINHANDTOPROVIDETHECORRECTORDERORDERBY,ONTHEOTHERHAND,WORKSAFTERWHEREBUTBEFORESELECTHOWDOIKNOWTHISTHEFOLLOWINGSTATEMENTWORKSSELECTNAMEFROMFOODSORDERBYIDIAMASKINGSQLITETOORDERBYACOLUMNTHATISNOTINTHERESULTTHEONLYWAYTHISCOULDHAPPENISIFTHEORDERINGTAKESPLACEBEFOREPROJECTIONWHILETHEIDCOLUMNISSTILLINTHESETWHILETHISWORKSINSQLITE,ITISALSOSPECIFIEDINSQL2003FUNCTIONSANDAGGREGATESRELATIONALALGEBRASUPPORTSTHENOTIONOFFUNCTIONSANDAGGREGATESTHROUGHTHEEXTENDEDOPERATIONKNOWNASGENERALIZEDPROJECTIONTHESELECTCLAUSEISAGENERALIZEDPROJECTIONRATHERTHANJUSTAFUNDAMENTALPROJECTIONTHEFUNDAMENTALPROJECTIONOPERATIONONLYACCEPTSCOLUMNNAMESINTHEPROJECTIONLISTASAMEANSTOPRODUCEACOLUMNWISESUBSETGENERALIZEDPROJECTIONACCEPTSTHISASWELLASARITHMETICEXPRESSIONS,FUNCTIONS,ANDAGGREGATESINTHEPROJECTIONLIST,INADDITIONTOOTHERFEATURESSUCHASGROUPBYANDHAVING,ALLOFWHICHARECOVEREDHEREANDINTHESUBSEQUENTSECTIONSSQLITECOMESWITHVARIOUSBUILTINFUNCTIONSANDAGGREGATESTHATCANBEUSEDWITHINVARIOUSCLAUSESFUNCTIONTYPESRANGEFROMMATHEMATICALFUNCTIONSSUCHASABS,WHICHCOMPUTESTHEABSOLUTEVALUE,TOSTRINGFORMATTINGFUNCTIONSSUCHASUPPERANDLOWER,WHICHCONVERTTEXTTOUPPERANDLOWERCASE,RESPECTIVELYFOREXAMPLESQLITESELECTUPPERHELLONEWMAN,LENGTHHELLONEWMAN,ABS12UPPERHELLONEWMANLENGTHHELLONEWMANABS12HELLONEWMAN1212NOTICETHATTHEFUNCTIONNAMESARECASEINSENSITIVEIE,UPPERANDUPPERREFERTOTHESAMEFUNCTIONFUNCTIONSCANACCEPTCOLUMNVALUESASTHEIRARGUMENTSSQLITESELECTID,UPPERNAME,LENGTHNAMEFROMFOODSWHERETYPE_ID1LIMIT10IDUPPERNAMELENGTHNAME1BAGELS62BAGELS,RAISIN143BAVARIANCREAMPIE184BEARCLAWS105BLACKANDWHITECOOKIES236BREADWITHNUTS177BUTTERFINGERS138CARROTCAKE119CHIPSAHOYCOOKIES1810CHOCOLATEBOBKA15SINCEFUNCTIONSCANBEAPARTOFANYEXPRESSION,THEYCANALSOBEUSEDINTHEWHERECLAUSESQLITESELECTID,UPPERNAME,LENGTHNAMEFROMFOODSWHERELENGTHNAMESELECTCOUNTFROMFOODSWHERETYPE_ID1COUNTTHECOUNTAGGREGATERETURNSACOUNTOFEVERYROWINTHERELATIONWHENEVERYOUSEEANAGGREGATE,YOUSHOULDAUTOMATICALLYTHINK,“FOREACHROWINATABLE,DOSOMETHING”ITISTHECOMPUTEDVALUEOBTAINEDFROMDOINGSOMETHINGWITHEACHROWINTHETABLEFOREXAMPLE,COUNTMIGHTBEEXPRESSEDINTERMSOFTHEFOLLOWINGPSEUDOCODEINTCOUNTCOUNT0FOREACHROWINRELATIONCOUNTCOUNT1RETURNCOUNT中文翻译现在让我们来看一个实际操作的例子。考虑以下几点SELECTNAME,TYPE_IDFROMFOODS在资料中执行这个方案，选择他们三纵列中的两列姓名和ID号。这个ID列被显示出来，正如图45所示。图45设计的资料让我们总结一下FROM子句带来了输入的关系和执行一个链接，从而使他们结合成R1的单一的关系。在WHERE子句接受R1以及经过约束筛选它，产生一个新的关系R2。SELECT子句接受R2并且执行规划，产生最后的结果。这个过程显示在图46中。图46在SELECT命令中的约束和规划举一个简单的例子，你能开始了解怎样操作一个常规的和SQL查询语言在他们相关操作期间的的操作。这是一个在真实的数学下的筛选。筛选如果说SELECT命令在SQL中是最复杂的命令的话，那么这个WHERE子句是最复杂的子句在SELECT命令下。接着，正如SELECT命令进入了集合论观点，WHERE子句也进入了正式的逻辑性观点。大体上，WHERE子句通常是SELECT命令的一个部分最复杂子句。但是它也做了大部分的工作。对于它的结构有一个固定的理解将很有可能带来最全面的回报在你逐日的运用SQL的过程中。数据库应用WHERE子句通过FROM子句和每行产生的关系。按照早期的规定，WHERE约束筛选。WHERE的论点是一个合乎逻辑的阐述。这个阐述，在最简单的认识上，仅仅是关于某事的一个声明。考虑到以下声明THEDOGSUBJECTISPURPLEANDHASATOOTHYGRINPREDICATE狗（主语）是紫色的而且漏齿而笑（谓语）。狗是主语，谓语由两部分组成（颜色是紫色和漏齿笑是漏出牙齿）。这部分陈述可能是真实的也可能是错误的，依赖谓语被应用到狗。形式逻辑术语，一段由主语和谓语组成的陈述叫一个命题。所有的命题不是对的就是错的。谓语说的是关于主语的一些事。主语和WHERE子句是一行。这行是逻辑主语。WHERE子句是逻辑谓语。共同形成一个逻辑命题，哪一个估计是正确或是错误的。这个命题明确的表达和估计在R1中。每行的那个命题真正结果包含的R2中。每行错误的那个被排除。因此狗这个命题被翻译为像这样的一个相同意思的关系SELECTFROMDOGSWHERECOLORPURPLEANDGRINTOOTHY数据库将会获取每行狗的叙述和应用WHERE子句来形成逻辑命题THISROWHASCOLORPURPLEANDGRINTOOTHY这也适应于特定的行，或是错误的无关紧要的。如果它是正确的那么这个行（或是狗）确实是紫色的和多齿的，并且它包含在结果当中。如果这个命题是错误的，那么这个行可能是紫色的但不是多齿的，或是多齿的但不是紫色的，或既不是紫色的也不是多齿的。在任何的案例中，这个命题是错误的并且这个行因此而被排斥。WHERE是一个强大的筛选器。它在很大程度上提供你控制包含（或是排斥）的行（或是来自）的结果。作为一个逻辑谓语，它也是一个逻辑语法。一个逻辑语法是一个用来评价两种可能中的一个结果的语法正确或是错误。一个逻辑谓语因而仅仅是一个逻辑语法中的特定的方法用来限制来自主语的命题。在他们中最简单的，逻辑语法与关系运算符相比由两个值或是值表现所组成。值的表现根据值和运算符来建立。一个关系运算符在这里提到的是在数学方面的关系运算符。它不同于在关系认识上的运算符在相关的理解中，作为关系代数的定义。一个关系运算符在数学上是一个叙述两个值和评价对错的运算符（EG,XY,XY）。一个关系运算符在关系代数中是一个运算符它带走两个或是更多的关系和产生一个新关系。为了使混淆减少到最低，这个讨论的范围被局限于一个相关的操作约束当作被执行的WHERE子句在SQL中。这个WHERE子句在逻辑命题中被表达，运用关系性的（数学性的思维）运算符。对于这部分的剩余，“关系运算符”特别的以数学的思维提到。值一个运算符带走一个或是更多的值作为输入并且生产出一个值作为输出。一个运算符是因此被命名的因为它执行了各种的运算，生产出各种的结果。二进制运算符是两个输入值（或是操作数）的运算。十进制运算符是用三个操作数，一元的运算符进行一次运算，等等。许多运算符生产出相同的信息它们消耗（运算符运算的数量和生产的数量，等等）这样的运算符能被排列在一起，输送这个输出一个运算符给另一个的输入（图47），形成值的表现。图47一元，二进制，和十进制运算符被排列在一起的运算符，你能创造值的表现在这期间然而其他的值的表现会任意的复杂化。例如XCOUNTEPISODESNAMEYCOUNTFOODSNAMEZY/X11二进制运算符二进制运算符截至目前为止在SQL所有运算符中是最普遍的运算符。表42列出二进制运算符在SQLITE中的优先权，从最高到最低。运算符在每一个颜色的组里都有平等的优先权。优先权的测量是随机的顺序赋值给一个表达式给多样的运算符。例如，用表达式437它的值是25这个乘法运算符比加法运算符拥有更高的优先级，并且因此先求出值。所以这个表达式被计算成（37）4优先级能够运用圆括号。这个表达式（43）7不是25。这里，这个括号已经声明一个外在的运算顺序。表格42二进制运算符运算符（例如，加法，减法，除法）是二进制运算符获得数值和生产出一个数值。关系运算符（例如，,）是二进制运算符的比较值和值的表达并且返回一个逻辑值（也叫做真值），它既不是正确的也不是错误的。关系运算符来自逻辑表达式，例如X51SELECT12120SQLITESELECT1SELECT12120SQLITESELECT1AND11AND11逻辑运算符逻辑运算符（AND,OR,NOT,IN）实际上是二进制运算符对值或逻辑的表达。它们的结果依赖于它们输入的特定的值。它们习惯于从简单的表达式中创建出复杂的表达式，例如X5ANDX3Y4ANDNOTY0COLORPURPLEANDGRINTOOTHY真值的产生是逻辑运算符为了依赖运算符的一对特定的观点。例如，逻辑AND需要两个输入值是真而返回真，逻辑OR，另一方面，仅仅需要一个输入值是正确的则返回真。全部可能的结果给出了特定的定义在逻辑运算真值表中。逻辑运算真值表为AND和OR分别展示在表43和44。表43真值表与运算表44真值表或运算这是用来形成WHERE子句的资料。利用逻辑运算符，你能创建一个复杂的逻辑谓语。谓语是如何定义WHERE子句关系约束操作约束。例如表达SQLITESELECTFROMFOODSWHERENAMEJUJYFRUITANDTYPE_ID9IDTYPE_IDNAME2449JUJYFRUIT这个约束依照NAMEJUJYFRUITANDTYPE_ID9表达，由两个逻辑表达式加一个逻辑AND组成。包含的结果中这两种情况必须适用于任何一种情况。运算符LIKELIKE是一独特有用的一种关系运算符。LIKE相当于号，用于查找指定列中于匹配串常量匹配的元组。例如，从全部行中选择谁的名字是从字母J开始的，你可以做下列各项SQLITESELECTID,NAMEFROMFOODSWHERENAMELIKEJIDNAME156JUICEBOX236JUICYFRUITGUM243JELLOWITHBANANAS244JUJYFRUIT245JUNIORMINTS370JAMBALAYA百分比符号在指定字符串中匹配0个或多个字符。下划线符号在指定字符串中匹配任意一个字符。百分比符号是贪婪的。它将会除去除了特定字符之外的每个字符。如果它的特定行的两端左边或是右边，它将会删掉每个字符在各自的边上。考虑下列例子SQLITESELECTID,NAMEFROMFOODSWHERENAMELIKEACPIDNAME127GUACAMOLEDIP168PEACHSCHNAPPS198MACKINAWPEACHES另一个使用方法是用NOT否定样式SQLITESELECTID,NAMEFROMFOODSWHERENAMELIKEACPANDNAMENOTLIKESCHIDNAME38PIEBLACKBERRYPIE127GUACAMOLEDIP198MACKINAWPEACHES限度和排序你能用LIMIT和OFFSET关键字来限制结果中特定列的和大小。LIMIT指定返回记录中的最大值。OFFSET指定跳过记录中的数值。例如,下面陈述获得的资料是用LIMIT和OFFSETSELECTFROMFOOD_TYPESLIMIT1OFFSET1ORDERBYIDOFFSET子句跳过一行THEBAKERYROW和LIMIT子句返回一行中的最大值THECEREALROW但是那有一种别的方法更好ORDERBY。这个子句以列或者列前返回分类的结果。这个原因是重要的在这个例子中，因为从行中返回从SELECT是根本不能保证在顺序在SQL标准声明中。因而，ORDER子句本质上是能依赖结果开始排列特定的顺序。ORDERBY子句的语法和SELECT子句的语法是相似的在每行中是用逗号分离开的。每个记录都有资格排列顺序ASC（升序）或DESC（降序）。例如SQLITESELECTFROMFOODSWHERENAMELIKEBORDERBYTYPE_IDDESC,NAMELIMIT10IDTYPE_IDNAME38215BAKEDBEANS38315BAKEDPOTATOW/SOUR38415BIGSALAD38515BROCCOLI36214BOUILLABAISSE32812BLT32712BACONCLUBNOTURKE32612BOLOGNA32912BRISKETSANDWICH27410BACON代表你仅仅需要从ORDERBY第二个（三个，等等）列时复制值在第一个（二个，等等）的顺序列中。在这里，那有复制的TYPE_IDS我应该集合他们在一起，按字符的顺序排列组里的资料。如果你能一起使用LIMIT和OFFSET，你能用逗号在OFFSET关键字那个地方。例如，下面的SQL语句SELECTFROMFOODSWHERENAMELIKEBORDERBYTYPE_IDDESC,NAMELIMIT1OFFSET2能相等的被表达于SQLITESELECTFROMFOODSWHERENAMELIKEBORDERBYTYPE_IDDESC,NAMELIMIT1,2IDTYPE_IDNAME38415BIGSALAD这里，逗号后有LIMIT1加OFFSET子句。