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PART2Chapter8ApplicationDesignandDevelopmentFig.1.0.6DataabstractionandSchema用户A1Specificationofuserrequirements(需求分析说明书)Fig.6.0.3DBAS生命周期模型Casestudyusedinthischapter§8.6Trigger(触发器)8.6Trigger(cont.)Supposethat,insteadofallowingnegativeaccountbalances,thebankdealswithoverdrafts(透支)byfollowingactionssettingtheaccountbalancetozerocreatingaloanintheamountoftheoverdraftgivingthisloanaloannumberidenticaltotheaccountnumberoftheoverdrawnaccountTriggeroverdraft-triggerinFig.8.8thetriggeringeventisupdateonbalancetheconditionforexecutingthetriggerisanmodificationtotheaccountrelationthatresultsinanegativebalancevaluecreatetriggeroverdraft-triggerafterupdateonaccount---eventreferencingnewrowasnrowforeachrowwhennrow.balance<0-------conditionbeginatomic-------actioninsertintoborrower(selectcustomer-name,account-numberfromdepositorwherenrow.account-number=depositor.account-number);insertintoloanvalues(nrow.account-number,nrow.branch-name,–nrow.balance);updateaccountsetbalance=0whereaccount.account-number=nrow.account-numberendGSM网络配置数据库中，利用触发器实现向小区(cell)中新增频点，约束：每个小区最多只有8个频点计算性能指标“BSC切换成功率”，原理：每当发生一次BSC切换时，根据本次切换是否成功重新计算“切换成功率”createtriggeroverfreonCell_TCHforinsert/*事件event*/asIFexists(selectcellid,count(TCHno)asnum/*条件condition*/fromCell_TCHgroupbycellidhavingcount(TCHno)>8)BegintransactiondeletefromCell_TCHwhereCell_TCH.TCHno=(selectmin(TCHno)asminfrenofromCell_TCHgroupbycellidhavingcount(TCHno)>8)print'插入TCH频点多于8个，频点号最小的已经被删除！'Endasinglestatementcanthenbeusedtocarryoutmultipleactionsonthebasisofthetemporary/transitiontables,andthusismoreefficientwhendealingwithSQLstatementsthatupdatealargenumberofrowsAlthoughtriggercanbeusedtomaintainDBSintegrity,itmaybeinefficientinsomecasesifeventsoccursfrequently,actionswillbefrequentlytakentoaccessonDBSomemoreefficientmechanisms,suchasmaterializedviews,insteadoftriggers,areusedtomaintaindatainDBFormoredetails,refertoAppendixAWhenNotToUseTriggers§8.7AuthorizationinSQLFig.SecurityControlArc
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