sql: aggregation, joins, and triggers csc 343 winter 2018 · grouping we may follow a select...

SQL:Aggregation,Joins,andTriggersCSC343Winter2018MICHAEL L IUT(MICHAEL.L [email protected])DEPARTMENT OF MATH EMAT ICA L ANDCOMPUTAT IONA L SC IENCE SUN IV E RS IT Y OF TORONTO M ISS ISSAUGA

AggregationOperators

Columnvaluesarecalculated,andasingle valueisreturned.

SUM,AVG,COUNT,MIN,andMAX.

These operationscanbeapplied onaSELECTclause inaquerytoproducetheaggregationonthecolumn.

e.g.COUNT(*)à countsthenumberoftuples inatable.

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AggregationOperationsareColumnOperations!

Example:Aggregation

FromSells(bar, beer,price), findtheaveragepriceofBud.

SELECTAVG(price)

FROMSells

WHEREbeer=‘Bud’;

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AggregationOperators

Anaggregationoperatormaynotappear intheWHEREclause unless itisinasubquerycontained inaHAVINGclauseoraSELECT list,andthecolumnbeingaggregated isanouterreference.

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Anexamplewillshortlyfollow.

DuplicatesinanAggregationToeliminate duplicates useDISTINCT insideanaggregation.

Example: Findthenumberofdifferent priceschargedforBud.

SELECT COUNT(DISTINCTprice)

FROMSells

WHERE beer=‘Bud’;

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NULLValuesinAggregation

NULLvaluesareNEVER contributed toaSUM,AVG,orCOUNT.

NULLvaluescanNEVER betheMINorMAXofacolumn.

However,ifallthevalues inthecolumnareNULL,thentheresultoftheaggregation isalsoNULL.

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Exception: The COUNTofanemptyset is0.

Example:TheNULLEffectSELECTCOUNT(*)

FROMSells


SELECTCOUNT(price)

FROMSells


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Thenumber of barsthatsellBud.

Thenumber of barsthatsellBudataknown price.

i.e.where theprice isNOTNULL.

Recall: Sells(bar,beer,price).

Example:ASimpleQuery

Findtheageoftheyoungestemployeeateachratinglevel.

SELECT MIN(age)

FROMEmployees

WHERErating=i;

Note: “i”represents aratingvalue.

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Grouping

WemayfollowaSELECT-FROM-WHERE expression byGROUPBYandalistofattributes.

Therelationthatresults fromtheSELECT-FROM-WHERE isgroupedaccordingtothevaluesofallthose attributes, andanyaggregation isapplied onlywithineachgroup.

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SELECT rating,MIN(age)FROM EmployeesGROUPBYrating;

Example:Grouping

FindtheaveragepriceforeachbeerfromSells(bar, beer,price).

SELECT beer,AVG(price)

FROMSells

GROUPBY beer;

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beer AVG(price)Bud 2.33Miller 4.55… …

Result:

SELECTclauseRestrictionswithAggregation

Tocontinuewiththeparagraphonslide#4:◦ Ifanyaggregation isused,theneachelement oftheSELECT listmustbeeither:

1. Aggregated,or2. AnattributeontheGROUPBY list.

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Example:GroupingForeachdrinker, findtheaveragepriceofBudatthebarstheyfrequent.Youwilluse:Sells(bar,beer,price) andFrequents(drinker, bar) forthisquery.

SELECT drinker, AVG(price)

FROMFrequents, Sells

WHEREbeer=‘Bud’ANDFrequents.bar =Sells.bar

GROUPBYdrinker;

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Compute alldrinker-bar-pricetriples for ‘Bud’.

Then group themby drinker.

IllegalQueryExample

SELECT bar,beer,MIN(price)

FROMSells

GROUPBY bar;

Thisqueryisillegal inSQL.

There isonlyonetuple outputforeachbar.Thus,nouniquewaytoselectwhichbeertooutput

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IllegalQueryExampleInDetail

SELECT bar,beer,MIN(price) ASminPriceFROMSellsGROUPBY bar;

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bar beer minPrice

Joe ? 3.00

Tom ? 3.50

Jane ? 3.25

Result:

{Bud, Miller,Coors}?

bar beer price

Joe Bud 3.00

Joe Miller 4.00

Tom Bud 3.50

Tom Miller 4.25

Jane Bud 3.25

Jane Miller 4.75

Jane Coors 4.00

Only one tuple output foreachbar,no unique waytoselect

which beer tooutput.

Sells:

Ideally, wewouldwanttoGROUPBY

beer.

HAVINGClauses

HAVING<condition> mayfollowaGROUPBYclause.

Ifso,thecondition applies toeachgroup,andgroupsnotsatisfying theconditionareeliminated.

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Example:HavingFromSells(bar,beer,price) and Beers(name,manf),findtheaveragepriceofthebeersthatareeitherservedinatleastthreebarsoraremanufacturesbyPete’s.

SELECTbeer,AVG(price)FROMSellsGROUPBYbeerHAVINGCOUNT(bar)>=3ORbeerIN(SELECTname

FROMBeersWHEREmanf =‘Pete”s’

);

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Beersmanufactured byPete’s.

Beergroupswith atleast3non-NULLbars.

RequirementsofHAVINGConditions

Anythinggoes inasubquery.

Outside subqueries, theymayrefertoattributes onlyiftheyareeither:

1. Agroupingattribute,or

2. Aggregated.

(samecondition asinSELECTclauseswithaggregation)

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AggregationOperatorsAnaggregationoperatormaynotappear intheWHEREclause unless itisinasubquerycontained inaHAVINGclause oraSELECT list,andthecolumnbeingaggregated isanouterreference.

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SELECT empID,SUM(amount)FROM SalesGROUPBYEmployeeHAVING SUM(amount)>20000;

OR

SELECT empID,SUM(amount)FROM SalesGROUPBYEmployeeWHEREempID IN(

SELECTMAX(empID)FROMEmployees);

Recall: AggregationOperationsareColumnOperations!

AFinalExample

SELECT bar,SUM(qty)ASsumQFROMSellsGROUPBY barHAVINGSUM(qty)>4;

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bar sumQ

Tom 5

Jane 6

Result:

bar beer price qty

Joe Bud 3.00 2

Joe Miller 4.00 2

Tom Bud 3.50 1

Tom Miller 4.25 4

Jane Bud 3.25 1

Jane Miller 4.75 3

Jane Coors 4.00 2

Sells:

CrossProduct

A.K.A.CartesianProduct.Denotedby:X

Evaluating joinsinvolvescombiningtwoormorerelations.

Giventworelations, S andR,eachrowofS ispairedwitheachrowofR.

Resulting Schema:oneattributefromeachattribute ofS andR.

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Example:CrossProduct

SELECT drinkerFROMFrequents,SellsWHERE beer=‘Bud’ANDFrequents.bar =Sells.bar;

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bar beer price

Joe Bud 3.00

Tom Miller 4.00

Jane Lite 3.25

Sells:

drinker bar

Aaron Joe

Mary Jane

Frequents: (bar) beer price drinker (bar)

Joe Bud 3.00 Aaron Joe

Joe Bud 3.00 Mary Jane

Tom Miller 4.00 Aaron Joe

Tom Miller 4.00 Mary Jane

Jane Lite 3.25 Aaron Joe

Jane Lite 3.25 Mary Jane

Sells X Frequents

drinker

Aaron

Result:

CrossProduct

Ingeneral:

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TableA TableB AXBRows N M N*M

Columns C K C+K

JoinedRelations

Joinoperations taketworelations andreturnsasaresultanotherrelation.

Ajoinoperation isaCartesian productwhichrequiresthattuples inthetworelationsmatch(undersomecondition). Italsospecifies theattributesthatarepresent intheresultofthejoin.

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Example:JoinOperations

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course_id title dept_name credits

BIO-301 Genetics Biology 4

CS-190 Game Design Comp Sci 4

CS-315 Robotics Comp Sci 3

Course:

course_id prereq_id

BIO-301 BIO-101

CS-190 CS-101

CS-347 CS-101

Prereq:

Observe:1. Theprerequisite information ismissing for CS-315.2. Thecourse information ismissing forCS-347.

TypesofJoins

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OuterJoin

Anextension ofthejoinoperationthatavoids lossofinformation.

SupposeyouhavetworelationsR andS.AtupleofR thathasnotuple ofSwithwhichitjoins issaidtobedangling.◦ Similarlyforatuple ofS.

Computes the joinandthenaddstuplesfromonerelationthatdoesnotmatchtuples intheotherrelationtotheresultofthe join.

Outerjoin preserves danglingtuples bypaddingthemwithNULL.

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LeftOuterJoin

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Course:

course_id prereq_id

BIO-301 BIO-101

CS-190 CS-101

CS-347 CS-101

Prereq:

course_id title dept_name credits prereq_id

BIO-301 Genetics Biology 4 BIO-101

CS-190 Game Design Comp Sci 4 CS-101

CS-315 Robotics Comp Sci 3 NULL

Result:

RightOuterJoin

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Course:

course_id prereq_id

BIO-301 BIO-101

CS-190 CS-101

CS-347 CS-101

Prereq:




CS-347 NULL NULL NULL CS-101

Result:

FullOuterJoin

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Course:

course_id prereq_id

BIO-301 BIO-101

CS-190 CS-101

CS-347 CS-101

Prereq:




CS-315 Robotics Comp Sci 3 NULL

CS-347 NULL NULL NULL CS-101

Result:

InnerJoin

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Course:

course_id prereq_id

BIO-301 BIO-101

CS-190 CS-101

CS-347 CS-101

Prereq:




Result:

Course INNER JOIN Prereq ONCourse.course_id =Prereq.course_id

OuterjoinsR OUTERJOINS isthecoreofanouter joinexpression.

Modified bythefollowing:1. OptionalNATURAL infrontofOUTER.

◦ Checkequality onallcommonattributes.◦ Notwoattributeswiththesame nameintheoutput.

2. OptionalON<condition> afterJOIN.3. OptionalLEFT,RIGHT,orFULLinfrontofOUTER.

◦ LEFT=paddanglingtuples ofR only.◦ RIGHT=paddanglingtuples ofS only.◦ FULL=padboth;this istheDEFAULTchoice.

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Example:OuterJoin

A B1 24 5

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A B C1 2 34 5 NULL

NULL 6 7

B C2 36 7

R: S:

Result:

RNATURALFULLOUTERJOINS

NOTE: (1,2) joins with(2,3), but theothertwotuples aredangling!

Triggers

Aprocedurethatisautomatically invokedbytheDBMSinresponse tospecified changestothedatabase.◦ Typically invokedbytheDBA.

Adatabasethathasasetofassociated triggersiscalledanactivedatabase.

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Triggers

Atriggerdescription contains threeparts:

1. Event:achangetothedatabase activatesthetrigger.2. Condition: aqueryortestthatisrunwhenthetriggerisactivated.3. Action:aprocedurethat isexecutedwhenthetrigger isactivatedand

itscondition istrue.

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Triggers

Atriggercanbethoughtofasa‘daemon’whichmonitorsadatabase, andisexecuted whenthedatabase ismodified inawaythatmatchestheeventspecifications.

i.e.INSERT,UPDATE,andDELETE statements canactivateatrigger.

Usersareoftenunawarethatatriggerwasexecutes asasideeffectoftheirprogram.

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DaemonAcomputerprogramthatrunsasabackgroundprocess insteadofbeingcontrolledbyaninteractive user.

e.g.SSHD isthedaemonthatserves incomingSSH connections.

This isseen incomputeroperatingsystems, specifically inmultitasking.

InUNIX,theparentprocessofadaemon isusuallythe init process.

NowbacktoTriggers…

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No, not the ancient Greek word thatrefers to benevolent or benignnatural spirits… I am talking aboutthem in the computing sense! J

Triggers

Acondition inatriggercanbeaBoolean expression oraquery.

e.g.Allemployees salaries arelessthan$100,000.

Aqueryis interpretedasTRUEiftheanswerset isnon-empty.◦ This invokestheactionassociated withthetrigger.

Aqueryis interpretedasFALSEifithasnoanswers.◦ Noaction isinvoked.

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ImportantIssuesAssociatedWithTriggers

Whenatriggeraction executes inarelation,wemustspecifyatwhatpointinthesequence ofevents itoccurs.◦ BECAREFUL!

Depending onwhatthetriggerdoes,wemaywanttomodifyifitsactionoccursbefore orafter changesaremadetorelations.

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Example: let’ssaythatwehaveastatement thatINSERTsrecordsintoaStudent’stable.◦ Thisactivates atriggerthatmaintains statistics onhowmanystudentsyoungerthan18yearsofageareinserted atonetime.

Doesthistriggerhappenbefore orafterthechange?

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Doesthistriggerhappenbefore orafterthechange?

Youhavedifferentoptions, forexample:

1. Atriggerthatinitializes avariableusedtocountthenumberofqualifying insertions shouldbeexecuted before.

2. Atriggerthatexecutes onceafter eachrecordis inserted.◦ Maybewerequirethevalues inthenewrecordtodeterminetheaction.

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Example:Triggers

Let’ssaythatwehaveastatement thatINSERTsrecordsintoaStudent’stable.◦ Thisactivates atriggerthatmaintains statistics onhowmanystudentsyoungerthan18yearsofageareinserted atonetime.

Nowlet’sassume thatwemustexamine theage attributeoftheStudenttabletodecidewhethertoincrement thecount.◦ Thetriggeringeventherewouldoccurforeachmodifiedrecord.

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Example:TriggersCREATE TRIGGER init_count BEFORE INSERTONStudents /*Event*/

DECLAREcountINTEGER;

BEGIN /*Action*/count:=0;

END

CREATE TRIGGER incr_count AFTER INSERTONStudents /*Event*/WHEN (new.age <18) /*Condition*/FOREACHROWBEGIN /*Action*/

count:=count+1;END

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Called arow-level trigger.

“new” isjust aninsertedtuple.

Note: Theinit_count triggerisexecutedonceperINSERT,thus FOREACHROWwasomitted, however, itwould be

considered astatement-leveltrigger.

Row-LevelTriggervs.Statement-LevelTrigger

Astatement-level triggerisactivatedonce.Arow-leveltrigger isactivated onceperiterationoftheloop.

e.g.UPDATEattribute

SETcolumnOne =columnOne +1;

Thestatement-level triggerwillbeactivatedonetime(evenifnorowsareupdated).Therow-leveltriggerwillbeactivatedmillions oftimes (dependent onthesize/iters).

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ImportantNoteonTriggers

Watchoutforchainactivations!!!

Depending onyourexecution oftheaction,youcouldpotentially activateanothertrigger.Youcouldpotentially evenactivatethesametrigger.◦ These arecalledrecursivetriggers.

Recursivetriggersaremostlyunpredictable because ofthecombination ofchainactivationsandorderwhichtheDBMSprocesses thissequence canbedifficulttounderstand.

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Constraintsvs. TriggersTriggersarecommonlyusedtomaintaindatabase consistency.◦ MustensurethatanIntegrityConstraint isnotmoreproperlysuited.

Constraints areeasiertounderstandastheyarenotoperationally defined.

Constraints preventinconsistency byanykindofstatement.

Constraints affordtheopportunityfortheDBMStooptimize.

Triggersallowustoimposegeneral (flexible) constraints

Triggersalertuserstounusualevents(reflected inupdatestothedatabase).

Triggerscangeneratealogofevents tosupportauditing andsecuritychecks.

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TriggerFlexibility

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TriggerFlexibility

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AlerttoUnusualEvents

Wemaywanttocheck ifacustomerplacinganorderhasmadeenoughpurchases inthepastmonthtoqualifyforanadditional discount.

◦ How?Implement atriggertocheckrecentpurchasesanddisplaysmessage.◦ IFTRUE: InformtheSalesClerk.◦ Purpose?ToUPSELLorXSELLadditional productsastheyarereceiving adiscount.

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SomeReviewQuestions1. Whatarenested queries?HowandwhenwouldyouusetheoperatorsIN,

EXISTS,UNIQUE,ANY,andALL?

2. WhatareNULLvalues?Aretheysupported intherelationalmodel?Howdotheyaffectthemeaningofqueries? CanprimarykeyfieldsofatablecontainNULLvalues?

3. Whatisatrigger?Whatareits3parts?Whatisthedifferencebetween arow-levelandstatement-level trigger?

4. Whatisgrouping?Whatisits interaction oftheHAVINGandWHEREclauses?Mention anyrestrictions thatmustbesatisfies bythefieldswhichappear intheGROUPBYclause.

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Questions?

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I’LL BE ANSWERING QUESTIONS NOW

Q A&

THANKS FOR LISTENING

Citations,ImagesandResourcesDatabaseManagement Systems(3rd Ed.),Ramakrishnan &Gehrke

Somecontentisbased offtheslidesofDr.Fei Chiang- http://www.cas.mcmaster.ca/~fchiang/

http://csharpcorner.mindcrackerinc.netdna-cdn.com/UploadFile/BlogImages/06112016031910AM/sql.png

https://lukaseder.files.wordpress.com/2015/10/venn.png?w=662

http://stackoverflow.com/questions/6319183/aggregate-function-in-sql-where-clause

http://www.sql-server-performance.com/2007/aggregate-may-not-appear-in-where-clause/

http://bioinfo.mbb.yale.edu/course/projects/talk-1/db07.html

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sql: aggregation, joins, and triggers csc 343 winter 2018 · grouping we may follow a select...

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