{ } Plan#for#Today# CS#133:#Databases# RelaKonal#Calculus# Rel.#Alg.#Compound#Operator:# Division# A B = x y B( x, y A)

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1 Planforoday CS133:Databases Spring2017 Lec8 2/9 SQL Prof.Bethrushkowsky EnhanceunderstandingofsemanKcsof conceptualqueryevaluakon Buildonunderstandingoftheroleofprimary keysandnullvaluesinqueries PracKcereadingandwriKngmorecomplex SQLqueries Rel.Alg.CompoundOperator: Division Usefulforexpressing forall querieslike: ind%names%of%sailors%who%have%reserved%allboats. ora/b,axributesofbaresubsetofaxributesofa Mayneedtouse project operatorfirst E.g.,letAhave2fields,xandy;Bhasonlyfieldy: { } A B = x y B( x, y A) A/Bcontains(all(x(tuples(such(that(for(every(y(tuple( in(b,(there(exists(a(tuple(x,y(in(a RelaKonalCalculus uplerelakonalcalculus: Variablesrangeover(i.e.,getboundto)tuples Answer(tuples:anassignmentofconstantsto variablesthatmake(an(expression(evaluate(to(true( (.. EffecKvelytheprojectedaXributes EveryrelaKonalalgebraquerycanbeexpressed asasafecalculusquery,andviceversa CheckoutSecKon4.3inthebookformore!%

2 Logical%Query%PlanExample Example:collegedatabase Students(SID,name,gpa) Enrolled(SID,CID,grade) SELEC S.name, E.CID ROM Students S, Enrolled E WHERE S.SID=E.SID; Setsoftuples flowupward% Gettuplesfrom Students Pulloutname%% andcidfields Combine Gettuplesfrom Enrolled RelaKonalalgebra expression? SQL:StructuredQueryLanguage RelaKonalalgebraandcalculusformthebasisforSQL hestandardquerylanguagesupportedbymost commercialdbms SpecificaKon:originallyIBM,thenANSIstarKng1986 IBMSSystemR ANSISQL89 ANSI(SQL(92(( ANSI(SQL(99( ANSISQL2003 ANSISQL2008 ANSISQL2011 BasicSQLQuery SELEC[DISINC]targetAlist ROMrelaBonAlist [WHEREqualificaBon] ORDER(BY(field(s)[ASC DESC]( LIMI(num_rows QuerySemanKcs SemanKcsofanSQLqueryaredefinedintermsofthe followingconceptual(evaluajon(strategy: 1.doROMclause:computecrossAproductoftables (e.g.,studentsandenrolled). 2.doWHEREclause:CheckcondiKons,discardtuplesthat fail.(i.e., seleckon ). 3.doSELECclause:Deleteunwantedfields. (i.e., projeckon ). 4.IfDISINCspecified,eliminateduplicaterows. Notnecessarilyanefficientwaytocomputeaquery! AnopKmizerwillfindmoreefficientstrategiestogetthe same%answer.

3 Anytuple VisualizingQueryEvaluaKon SELECsname ROMSailors,Reserves WHERESailors.sid=Reserves.sidANDbid=103 JoincondiKon:arethesethesamesid? sid( sname( rajng( age( Ifso,returnthissname sid( bid( day( Isthisbid103? Anytuple Conceptually,%this%happens% for%all%pairs%of%tuples%% ExampleRelaKonInstances Wewillusethese instancesof relakonsinour examples. sid bid day /10/ /12/96 Reserves sid sname rating age 22 Dustin Lubber Bob Sailors bid bname color 101 Interlake blue 102 Interlake red 103 Clipper green 104 Marine red Boats (Assume%appropriate%foreign%key%constraints%are%used)% RangeVariables Canassociate range(variables withtherelakonsin theromclause saveswrikng,makesquerieseasiertounderstand likeanalias SELEC S.sname ROM Sailors S, Reserves R WHERE S.sid=R.sid AND bid=103 RangeVariables(cntd) Examplewhererangevariablesarerequired (selfrjoinexample): SELEC S1.sname, S1.age, S2.sname, S2.age ROM Sailors S1, Sailors S2 WHERE S1.age = S2.age AND S1.rating > S2.rating; Neededwhenambiguitycouldarise forexample,ifsamerelakonusedmulkplekmes insameromclause(calleda selfrjoin )( CouldtheresultcontainapairofSailorsthatare actually%the%same%person?

4 NullValues ieldvaluesinatuplearesomekmesmissing %(e.g.,arakngorgradehasnotbeen assigned) inapplicable%(e.g.,nospouse sname). SQLprovidesaspecialvaluenull(forsuchsituaKons. NullValues 3ValuedLogic Weneeda3qvaluedlogic. Values:rue,alseand Meaningofconstructsmustbe definedcarefully (e.g.,whereclauseeliminatesrows thatdonotevaluatetotrue.) (null > 0) (null + 1) (null = 0) null AND true NO hepresenceofnullcomplicatesthings.e.g.: Is rabng%>%8 trueorfalsewhenrabngisnull? WhataboutAND,ORandNO? Checkifavalueis/isnotnullusingISNULL AND Null NULL OR Null NULL Expressions CanusearithmeKcexpressionsinSELECclause (plusothercalculakonswe lldiscusslater) UseAStoprovidecolumnnames SELEC S.sname, S.rating % 2 AS evenoroddrating ROM Sailors S WHERE S.age >= 18; CanalsohaveexpressionsinWHEREclause: SELEC S1.sname AS name1, S2.sname AS name2 ROM Sailors S1, Sailors S2 WHERE 2*S1.rating > S2.rating; Query:indsidsofsailorswho ve reservedaredoragreenboat SELEC DISINC R.sid Why%do%we%want%% ROM Boats B,Reserves R DISINC%here?% AND (B.color= red OR B.color= green ); UNION:computetheunionofanytwounionAcompaBblesets oftuples(whicharethemselvestheresultofsqlqueries) (note: UNION eliminates duplicates bydefault. Overridew/ UNIONALL) SELEC R.sid ROM Boats B, Reserves R AND B.color= red UNION SELEC R.sid ROM Boats B, Reserves R AND B.color= green ;

5 Query:indsidsofsailorswho ve reservedaredandagreenboat IfwesimplyreplaceORbyANDinthepreviousquery, wegetthewronganswer.(why?) SELEC DISINC R.sid ROM Boats B,Reserves R AND (B.color= red AND B.color= green ) redandagreenboat(cntd) INERSEC: Discussedintextbook. Canbeusedtocompute theinterseckonofanytwo unionacompabblesetsof tuples. Alsointextbook:EXCEP (somekmescalledminus) IncludedintheSQL92 standard, butmanysystemsdon t supportthem. SELEC R.sid ROM Boats B, Reserves R AND B.color= red INERSEC SELEC R.sid ROM Boats B, Reserves R AND B.color= green redandagreenboat(cntd) What swrongwiththisversionofthepreviousquery? SELEC S.sname ROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color= red INERSEC SELEC S.sname ROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color= green NestedQueries CanuseSQLqueriestoaidtheevaluaKonof another%sql%query% WHEREclausecanitselfcontainanSQLquery! Actually,socanROMandHAVINGclauses. Example: SELEC S.sid ROM Sailors S WHERE S.rating > (SELEC AVG(rating) ROM Sailors); Howmanyresultsdoes thissubqueryreturn?

6 NestedQueries SubqueriescanalsoberelaKonswithmanytuples Names%of%sailors%who ve%reserved%boat%103:% SELEC S.sname ROM Sailors S oragiventupleinthe WHERE S.sid IN ( SELEC R.sid outerquery,checkif ROM Reserves R sid==anyresulttuple WHERE R.bid=103) fromtheinnerquery MoreonSetqComparisonOperators SQLoperatorstofiltertuplesbyapplyingtoarelaKonRto getaboolean(result( value%inr:%trueiffvalue%isequaltooneofthevaluesinunary%r EXISSR:%trueiffRisnotempty UNIQUER:%trueiffR%hasnoduplicates(orisempty) value%<op>anyr:trueiffvalue<op>somevalueinunaryr% value%<op>allr:%trueiffvalue<op>allvaluesinunaryr% SemanKcsofnestedqueries: hinkofanested%loopsevaluakon:or%each%sailors%tuple,% check%the%qualificabon%by%compubng%the%subquery% % ofindsailorswhohavenotreserved103,useno(in SELEC * ROM Sailors S WHERE S.age > ANY (SELEC S2.age ROM Sailors S2 WHERE S2.sname= Horatio ) Ingeneral,watchoutforaXributesthatcouldbeNULL! NestedQuerieswithCorrelaKon ind%names%of%sailors%who ve%reserved%boat%103:% SELEC S.sname ROM Sailors S WHERE EXISS (SELEC * ROM Reserves R WHERE R.bid=103 AND S.sid=R.sid) SubqueryrecomputedforeachSailorstuple. hinkofsubqueryasafunckoncallthatrunsaquery! What%if%we%replaced%EXISS%with%UNIQUE,%and%replaced% SELEC%*%with%SELEC%R.bid?% % SELEC S.sname ROM Sailors S WHERE UNIQUE (SELEC R.bid ROM Reserves R WHERE R.bid=103 AND S.sid=R.sid) RewriKngINERSECQueriesUsingIN ind%sids%of%sailors%who ve%reserved%both%a%red%and%a%green%boat:% SELEC R.sid ROM Boats B, Reserves R AND B.color= red AND R.sid IN (SELEC R2.sid ROM Boats B2, Reserves R2 WHERE R2.bid=B2.bid AND B2.color= green ) Similarly,EXCEPqueriescanbereqwriXen usingnoin.

7 Whatdoesthisquerydo? SELEC S.sname ROM Sailors S WHERE NO EXISS (SELEC B.bid ROM Boats B WHERE B.color = green NamesofSailorsS suchthat AND NO EXISS (SELEC R.bid ROM Reserves R WHERE R.bid = B.bid AND R.sid = S.sid)); thereisno greenboatb Division!!Namesofsailorswhohavereservedallgreenboats withoutareservakon betweenthissailorand thatboat