LimitRowsLimitRows Copyright Copyright?2013,Oracleand/orits.sThislessoncoversthefollowingApplySQLsyntaxtorestricttherowsreturnedfromaDemonstrateapplicationoftheWHEREclauseExinwhyitisimportant,fromabusiness,tobeabletoeasilylimitdataretrievedfromatableConstructandproduceoutputusingaSQLquerycontainingcharacterstringsanddatevaluesHaveyoueverhad“informationoverload”?The ison,yourMomisaskingyouhowschoolwenttoday,thephonerings,andthedogisbarking.Wouldn'titbenicetobeabletorestricttheamountofinformationyouhavetoprocessatonetime?InSQL,thisisthejoboftheWHEREItisimportanttobeabletochoosetheinformationyouneedtoseefromatable.Tablescanhavemillionsofrowsofdata,anditisawasteofresourcestosearchandreturndatayoudon'tneedorwant.SELECTYouuseSELECTtoretrieveinformationfromthedatabase.ASELECTstatementmustincludeataminimumaSELECTclauseandaFROMclause.TheWHEREclauseisoptional.SELECT*|{[DISTINCT]SELECT*|{[DISTINCT]column|expressionFROM[WHEREWHEREWhenretrievingdatafromthedatabase,youmayneedtolimittherowsofdatathataredisyed.YoucanplishthisusingtheWHEREclause.AWHEREclausecontainsaconditionthatmustbemet,anditdirectlyfollowstheFROMclauseinaSQLstatement.ThesyntaxfortheWHEREclauseWHEREWHEREcolumn_namecomparison_condition iascannotbeusedintheWHEREWHEREClauseExaminethefollowingSQLstatementfromtheDJsonDemanddatabase:SELECTSELECTid,first_name,last_name NotehowbyaddingaWHEREclause,therowsarelimitedtothoserowswherethevalueofIDis22.SELECTSELECTfirst_name,last_name,expertise WHEREid=ComparisonOperatorsintheWHEREAsyousawonthepreviousslide,the=signcanbeusedintheWHEREclause.Inadditiontothe“equalto”operator(=),othercomparisonoperatorscanbeusedtocompareoneexpressiontoanother: equal greater>=greaterthanorequal less<=lessthanorequal<>notequalto(or!=orLimitLimitRows Copyright Copyright?2013,Oracleand/orits.sComparisonOperatorsintheWHEREClauseIntheexamplebelow,whichcolumnnameisusedintheWHEREclause?Whatcomparisonoperatorisused?Is"90"acolumnnameoraconstant?SELECTSELECTemployee_id,last_name,department_id WHEREdepartment_id=90LimitRowsLimitRows Copyright Copyright?2013,Oracleand/orits.s ndDateStringsintheWHERECharacterstringsanddatesintheWHEREclausemustbeenclosedinsinglequotationmarks''.ButthisdoesNOTmakethemli lstrings.Theyarestilljustcharacteranddatestrings.Numbers,however,shouldnotbeenclosedinsinglequotationmarks. ndDateStringsintheWHEREClause(cont.)SELECTfirst_name,WHERElast_name= SELECTfirst_name,WHERElast_name= ndDateStringsintheWHEREClause(cont.)WhatdoyouthinkwillhappeniftheWHEREclausewrittenWHEREWHERElast_name=Allcharactersearchesarecase-sensitive.BecausetheD_Stablestoresallthelastnamesinthepropercase,norowsarereturnedinthiample. ndDateStringsintheWHEREClause(cont.)Thisisanimportantpointtoremember.Inanotherlesson,youwilllearntouseotherSQL LOWER,andINITCAPthatwillmakeiteasiertoavoidacase-sensitivemistake.LimitRowsComparisonOperatorsintheWHEREComparisonoperatorscanbeusedinallofthewaysintheWHERE='01-JAN->=cd_title'WhiteInthefollowingexamplefromtheDJsonDemanddatabase,whichrowswillbeselected?Willsalariesof3000beincludedintheresultset?SELECTSELECTlast_name,salary WHEREsalary<= Copyright?2013,Oracleand/oritsaffiliates.sLimitRowsLimitRows Copyright Copyright?2013,Oracleand/orits.sInthislesson,youshouldhavelearnedhowApplySQLsyntaxtorestricttherowsreturnedfromaDemonstrateapplicationoftheWHEREclauseExinwhyitisimportant,fromabusiness,tobeabletoeasilylimitdataretrievedfromatableConstructandproduceoutputusingaSQLcontainingcharacterstringsanddateThelinkforthelessonpracticeguidecanbefoundincourseresourcesinSectionSQLWHERESQL曾經遇到過“信息過載”？電視開著，你問你今天學校過得怎么樣，鈴響了，狗在吠叫。能夠限制一次必須處理的信息量不是很好嗎？在SQL中，這是WHERE子句的工作。SELECTSELECTSELECTSELECT和FROM子句。WHERE子句是可選的。WHEREWHERE此操作。WHERE子句包含必須滿足的條件，它直接位于SQL語句中的FROM子句之后。WHERE注意：WHEREWHERE子句（續檢查DJsonDemand數據庫中的以下SQLIDFIRST_NAME 33AllisonPlumb請注意，如何通過添加WHERE子句將行限制為ID值為22IDFIRST_NAME22JasonTsangWHEREWHERE等于大于大于或等于小于小于或等于不等于（或!=或^=）WHERE子句中的比較運算符（續在下面的示例中，WHERE子句中使用了哪個列名？使用什么比較運算符？“90”？WHEREWHEREWHERE子句中的字符和日期字符串（續請查看DJsonDemand數據庫中的以下示例。WHEREWHERE（續）如果WHERE子句寫成：所有字符搜索都區分大小寫。由于D_S表以正確的大小寫形