SQL: Queries, Programming, Triggerspages.cs.wisc.edu/~jignesh/cs564/notes/lec09-SQL.pdf · SQL...

SQL:Queries,Programming,Triggers

Chapter5,CowBookor

Seehttp://sqlzoo.net/

3/25/17 CS 564: Database Management Systems, Jignesh M. Patel 1

SQLLanguage

• DDL:Datadefinitionlanguage• DML:Datamanipulationlanguage• EmbeddedandDynamicSQL• Triggers• Security• TransactionManagement• RemoteDatabaseaccess


BasicSQLQuery

• Semantics/Conceptualevaluationstrategy:– Computethecross-productofrelation-list.– Discardresultingtuplesiftheyfailqualifications.– Deleteattributesthatarenotintarget-list.– IfDISTINCTisspecified,eliminateduplicaterows.

• Notanefficientevaluationplan!(Optimzier picksefficientplans)


SELECT [DISTINCT] target-listFROM relation-listWHERE qualification

Optional

List of relations

Attr1 op Attr2 OPS: <, >, =, <=, >=, <>

Combine using AND, OR, NOT

Attributes from input relations

ExampleofConceptualEvaluation


SELECT S.name, A.hours FROM Senators S, Attendance A WHERE S.ssn = A.ssn and A.date = ‘24-Sept-2016’

ssn name email age income

11-111 Bob [email protected] 51 100.122-222 Jane [email protected] 54 130.133-333 Jane [email protected] 51 99.8

ssn date hours

11-111 12-Aug-2016 1.133-333 24-Sept-2016 4.1

(ssn) name email age income (ssn) date hours

11-111 Bob [email protected] 51 100.1 11-111 12-Aug-2016 1.1

33-333 24-Sept-2016 4.111-111 Bob [email protected] 51 100.1

22-222 Jane [email protected] 54 130.1 33-333 24-Sept-2016 4.1


22-222 Jane [email protected] 54 130.1 11-111 12-Aug-2016 1.1

33-333 Jane [email protected] 51 99.8 11-111 12-Aug-2016 1.1


Findsenatorswhoattendedthe‘24-Sept-2016’session


SELECT S.nameFROM Senators S, Attendance A WHERE S.ssn = A.ssn and date = ‘24-Sept-2016’

n Add DISTINCT to this query. Effect?n Replace S.name by S.ssn.

Effect of adding DISTINCT to this query

RA: pname (sdate=‘24-Sept-2016’(Senators wv Attendance))

n Equivalent SQL?n What is the schema of Senators ⋈ Attendance?

ANoteonRangeVariables• Neededonlyifthesamerelationappearstwiceinthe

FROM clause.


SELECT S.name, A.hoursFROM Senators S, Attendance A WHERE S.ssn = A.ssn and A.date = ‘24-Sept-2016’

It is good style,however, to always userange variables!

ORSELECT Senators.name, Attendance.hoursFROM Senators , Attendance WHERE Senators.ssn = Attendance.ssn and

Attendance.date = ‘24-Sept-2016’

ExpressionsandStrings

• Illustratesuseofarithmeticexpressionsandstringpatternmatching

• AS and= aretwowaystonamefieldsinresult.• LIKE isusedforstringmatching.`_’standsforanyonecharacter

and`%’standsfor0ormorearbitrarycharacters.• Collation:sortorderforcharactersets


SELECT S.name, S.age, age1=S.age+2, S.income/S.age AS iarFROM Senators SWHERE S.sname LIKE ‘Ja_%Doe’ORDER BY S.name

Findsenatorswhoattendedeitherthe‘24-Sept-2016’or‘25-Sept-2016’session

• UNION:Computetheunionoftwounion-compatible setsoftuples– Samenumber/typesoffields.

• Alsoavailable:INTERSECTandEXCEPT (WhatdowegetifwereplaceUNION byEXCEPT?)

• SQLoddities:duplicateswithunion,except,intersect– Default:eliminateduplicates!– UseALLtokeepduplicates


SELECT S.ssnFROM Senators S, Attendance AWHERE S.ssn = A.ssnand (A.date = ‘24-Sept-2016’ or

A.date = ‘25-Sept-2016’)

SELECT S.ssnFROM Senators S, Attendance AWHERE S.ssn = A.ssnand A.date = ‘24-Sept-2016’UNIONSELECT S.ssnFROM Senators S, Attendance AWHERE S.ssn = A.ssnand A.date = ‘25-Sept-2016’

• INTERSECT:Computetheintersectionofanytwounion-compatible setsoftuples.

• IntheSQL/92standard,butsomesystemsdon’tsupportit.


SELECT S.ssn, S.nameFROM Senators S, Attendance AWHERE S.ssn = A.ssnand A.date = ‘24-Sept-2016’ INTERSECTSELECT S.ssn, S.nameFROM Senators S, Attendance AWHERE S.ssn = A.ssnand A.date = ‘25-Sept-2016’

Find senators who attended both the ‘24-Sept-2016’ and ‘25-Sept-2016’ session

SELECT S.ssnFROM Senators S, Attendance A1,

Attendance A2WHERE S.ssn = A1.ssn and S.ssn = A2.ssnand A1.date = ‘24-Sept-2016’ and A2.date = ‘25-Sept-2016’

Key field! What happens if S.name is used

NestedQueries

• PowerfulfeatureofSQL– WHERE clausecanitselfcontainanSQLquery!– Actually,socanFROM andHAVING clauses

• Tofindsailorswho’venot reserved#103,useNOTIN• ConceptualEvaluation: nestedloops ForeachSailorstuple,check

thequalificationbycomputingthesubquery.


SELECT S.snameFROM Sailors SWHERE S.sid IN (SELECT R.sid

FROM Reserves RWHERE R.bid=103)

Find names of sailors who’ve reserved boat #103

Can you rewrite this to not use a nested query?

Sailors (sid, sname, rating, age)Reserves (sid, bid, day)Boats (bid, bname, color)

NestedQuerieswithCorrelation

• UNIQUE returnstrueiftherowappearsonlyonce


Find names of sailors who’ve reserved boat #103

SELECT S.snameFROM Sailors SWHERE EXISTS (SELECT *

FROM Reserves RWHERE R.bid=103 AND S.sid=R.sid)

SELECT S.snameFROM Sailors SWHERE EXISTS (SELECT UNIQUE R.bid

FROM Reserves RWHERE R.bid=103 AND S.sid=R.sid)Why R.bid?

n Illustrates why, in general, subquery must be re-computed for each Sailors tuple.

n EXISTS tests if the set is not empty

------------------------------------------------------------------Find names of sailors with exactly one reservation for boat #103

MoreonSet-ComparisonOperators• We’vealreadyseenIN,EXISTSandUNIQUE.Canalsouse

NOTIN,NOTEXISTSandNOTUNIQUE.• Alsoavailable:op ANY,op ALL,opis<,≤,>,≥,=,≠

• FindsailorswhoseratingisgreaterthanthatofsomesailorcalledHoratio:


(SELECT *FROM Sailors SWHERE S.ratin)> ANY (SELECT S2.rating

FROM Sailors S2WHERE S2.sname=‘Horatio’)

SELECT *FROM Sailors SWHERE S.rating > ANY (SELECT S2.rating

FROM Sailors S2WHERE S2.sname=‘Horatio’)

• Similarly,INTERSECT queriesre-writtenusingIN.


Find sailors (sid) who’ve reserved some boat for ‘24-Sept-2016’ but have no reservations for ‘09-Oct-2016’

SELECT S.sidFROM Sailors S, Reserves RWHERE S.sid=R.sid

AND R.day=‘24-Sept-2016’AND S.sid NOT IN

(SELECT S2.sidFROM Sailors S2, Reserves R2WHERE S2.sid=R2.sid AND R2.day=‘‘09-Oct-2016’)


SELECT S.sidFROM Sailors S, Reserves RWHERE S.sid=R.sid

AND R.day=‘24-Sept-2016’EXCEPT(SELECT S2.sidFROM Sailors S2, Reserves R2WHERE S2.sid=R2.sid

AND R2.day=‘‘09-Oct-2016’)

Rewriting ExceptQueries Using NOT IN

DivisioninSQL

WithoutEXCEPT:


Find sailors who’ve reserved all boats.

SELECT S.snameFROM Sailors SWHERE NOT EXISTS

((SELECT B.bidFROM Boats B)

EXCEPT(SELECT R.bidFROM Reserves RWHERE R.sid=S.sid))

(1)

SELECT S.snameFROM Sailors SWHERE NOT EXISTS (SELECT B.bid

FROM Boats B WHERE NOT EXISTS (SELECT R.bid

FROM Reserves RWHERE R.bid=B.bid

AND R.sid=S.sid))

(2)

Sailors S such that ...

there is no boat B without ...

a Reserves tuple showing S reserved B


AggregateOperators


COUNT (*)COUNT ( [DISTINCT] A)SUM ( [DISTINCT] A)AVG ( [DISTINCT] A)MAX (A) Can use DistinctMIN (A) Can use Distinct

SELECT AVG (S.age)FROM Sailors SWHERE S.rating=10

SELECT COUNT (*) FROM Sailors S

SELECT AVG ( DISTINCT S.age)FROM Sailors SWHERE S.rating=10

SELECT S.sname FROM Sailors SWHERE S.rating= (SELECT MAX(S2.rating) FROM Sailors S2)

single column

SELECT COUNT (DISTINCT S.name)FROM Sailors S

Findname&ageoftheoldestsailor(s)

• Thefirstqueryisillegal!(waitforGROUPBY.)

• Q3isallowedintheSQL/92standard,butnotsupportedinsomesystems


SELECT S.sname, MAX (S.age)FROM Sailors SSELECT S.sname, S.ageFROM Sailors SWHERE S.age =

(SELECT MAX (S2.age)FROM Sailors S2)

SELECT S.sname, S.ageFROM Sailors SWHERE (SELECT MAX (S2.age)

FROM Sailors S2)= S.age

How many tuples in the result?

GROUPBYandHAVING• Applyaggregatetoeachofseveralgroups oftuples• Findtheageoftheyoungestsailorforeachratinglevel

– Don’tknow:#ratinglevels,andratingvalues– Supposewedidknowthatratingvaluesgofrom1to10

• wecanwrite10queriesthatlooklikethis(!):


SELECT MIN (S.age)FROM Sailors SWHERE S.rating = i

For i = 1, 2, ... , 10:SELECT MIN (S.age), S.ratingFROM Sailors SGROUP BY S.rating

QueriesWithGROUPBYandHAVING

• The target-listcontains– Attributenames:mustbeasubsetofgrouping-list.– Termswithaggregateoperations(e.g.,MIN(S.age)).

• Thegroup-qualification– Musthaveasinglevaluepergroup


SELECT [DISTINCT] target-listFROM relation-listWHERE qualificationGROUP BY grouping-listHAVING group-qualification

How many tuples in the result?

ConceptualEvaluation• Cross-product->discardtuples->applyprojection

->partitionintogroupsusingthegrouping-list attributevalues->eliminategroupsthatdon’tsatisfythegroup-qualification

• Expressionsingroup-qualification haveasinglevaluepergroup!– Ineffect,anattributeingroup-qualification thatisnotanargumentofanaggregateopalsoappearsingrouping-list.(SQLdoesnotexploitprimarykeysemanticshere!)

• Oneanswertupleisgeneratedperqualifyinggroup.


Findtheageoftheyoungestsailorwithage>=18,foreachratingwithatleast2such sailors

• 2ndcolumnofresultisunnamed.(UseAS tonameit)


SELECT S.rating, MIN (S.age)FROM Sailors SWHERE S.age >= 18GROUP BY S.ratingHAVING COUNT (*) > 1

sid sname rating age 22 dustin 7 45.0 31 lubber 8 55.5 71 zorba 10 16.0 64 horatio 7 35.0 29 brutus 1 33.0 58 rusty 10 35.0

rating age 1 33.0 7 45.0 7 35.0 8 55.5 10 35.0

rating 7 35.0

Answer relation

rating

Foreachredboat,findthenumberofreservationsforthisboat


SELECT B.bid, COUNT (*) AS scountFROM Sailors S, Boats B, Reserves RWHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’GROUP BY B.bid

SELECT B.bid, COUNT (*) AS scountFROM Sailors S, Boats B, Reserves RWHERE S.sid=R.sid AND R.bid=B.bidGROUP BY B.bid Would this work?HAVING B.color = ‘red’ note: one color per bid

Findtheageoftheyoungestsailorwithage>18,foreachratingwithatleast2sailors(ofanyage)

• SubqueryintheHAVING clause• Comparethiswiththequerywhereweconsideredonlyratingswith2sailorsover18!


SELECT S.rating, MIN (S.age) AS MINAGEFROM Sailors SWHERE S.age > 18GROUP BY S.ratingHAVING 1 < (SELECT COUNT (*) FROM Sailors S2

WHERE S.rating=S2.rating)

Findratingsforwhichtheaverageageistheminimumoftheaverageageoverallratings

• Aggregateoperationscannotbenested!WRONG:


SELECT S.ratingFROM Sailors SWHERE AVG(S.age) =

(SELECT MIN (AVG (S2.age)) FROM Sailors S2)

SELECT Temp.rating, Temp.avgageFROM (SELECT S.rating, AVG (S.age) AS avgage

FROM Sailors SGROUP BY S.rating) AS Temp

WHERE Temp.avgage = (SELECT MIN (Temp.avgage) FROM Temp)

§ Correct solution (in SQL/92):

NullValues• Represent

– unknown (e.g.,ratingnotassigned)or– inapplicable (e.g.,nospouse’sname)

• Complicationswithnulls:– Operatorstocheckifvalueis/isnotnull.– Israting>8 trueorfalsewhenrating isnull?

• Answer:Evaluatetounknown

– WhataboutAND,ORandNOT connectives?• Need3-valuedlogic (true,falseandunknown)

– Notunknown=unknown

– WHEREclauseeliminatesrowsthatdon’tevaluatetotrue– Newoperators(inparticular,outerjoins)possible/needed.


p q p AND q p OR q

T T T TT F F TT U U TF T F TF F F FF U F UU T U TU F F UU U U U

OuterJoin


sid bid day 22 101 10/10/99

Reservessid sname rating age 22 dustin 7 45.0 58 rusty 10 35.0

Sailors

Select S.sid, R.bidFrom Sailors S NATURAL LEFT OUTER JOIN Reserves R

sid bid 22 101 58 null

Result Similarly:• RIGHT OUTER JOIN• FULL OUTER JOIN

EmbeddedSQL

• CallSQLcommandsfromahostlanguage(e.g.,C)program.– SQLstatementscanrefertohostvariables(includingspecialvariablesusedtoreturnstatus).

– Mustincludeastatementtoconnect totherightdatabase.

• SQLrelationsare(multi-)setsofrecords,withnoaprioriboundonthenumberofrecords.NosuchdatastructureinC.– SQLsupportsamechanismcalledacursor tohandlethis.


Cursors• Candeclareacursoronarelationorquerystatement(whichgeneratesarelation).

• Canopen acursor,andrepeatedlyfetch atuple thenmove thecursor,untilalltuples havebeenretrieved.– Specialclause,calledORDERBY,incursorqueriestocontroltheorderinwhichtuples arereturned.

– FieldsinORDERBYmustalsoappearinSELECTclause.

• Canalsomodify/deletetuple pointedtobyacursor


Cursorthatgetsnamesofsailorswho’vereservedaredboat,inalphabeticalorder

• CanwereplaceS.sname byS.sid intheORDERBYclause!– EverycolumnintheORDERBYclausemustappearintheSELECT

clause


EXEC SQL DECLARE sinfo CURSOR FORSELECT S.snameFROM Sailors S, Boats B, Reserves R WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’ORDER BY S.sname

IntegrityConstraints• AnICdescribesconditionsthateverylegalinstanceofarelationmust

satisfy.– Inserts/deletes/updatesthatviolateIC’saredisallowed.

• TypesofIC’s:Domainconstraints,primarykeyconstraints,foreignkeyconstraints,generalconstraints.

• Cancreatenewdomains,DomainConstraints:– CREATEDOMAINLegalRatingsINTEGERDEFAULT0

CHECK(VALUE>=1andVALUE<=10)– CreateTableSailor(…,ratingLegalRatings,…)– UnderlyingdomainisstillIntegersforcomparison

• Cancreatenewtypes:CREATETYPEAllRatingsasINTEGER– Underlyingdomainisnowanewtype.Can’tcomparewithINTEGERwithouta

cast.NoneoftheaggregatesonINTEGERworkonAllRatings


TableConstraints• MoregeneralICsthan

keyconstraints• Canusequeries

toexpressconstraint

• Constraintscanbenamed.


CREATE TABLE Sailors( sid INTEGER,sname CHAR(10),rating INTEGER,age REAL,PRIMARY KEY (sid),CHECK ( rating >= 1

AND rating <= 10 )CREATE TABLE Reserves( sname CHAR(10),bid INTEGER,day DATE,PRIMARY KEY (bid,day),CONSTRAINT noInterlakeResCHECK (`Interlake’ <>

( SELECT B.bnameFROM Boats BWHERE B.bid=bid)))

ConstraintsOverMultipleRelations• Awkward

&Wrong!• IfSailorsisempty,

thenumberofBoatstuplescanbeanything!

• ASSERTION istherightsolution;notassociatedwitheithertable


CREATE TABLE Sailors( sid INTEGER,sname CHAR(10),rating INTEGER,age REAL,PRIMARY KEY (sid),CHECK ((SELECT COUNT (S.sid) FROM Sailors S) +(SELECT COUNT (B.bid) FROM Boats B) < 100)

CREATE ASSERTION smallClubCHECK((SELECT COUNT (S.sid) FROM Sailors S) +

(SELECT COUNT (B.bid) FROM Boats B) < 100)

Number of boatsplus number of sailors is < 100

Triggers• Trigger:procedurethatstartsautomaticallyifspecifiedchangesoccurtotheDBMS

• Threeparts:– Event(activatesthetrigger)– Condition(testswhetherthetriggersshouldrun)– Action(whathappensifthetriggerruns)

• BeforeandAfterTriggers

• TriggerExecution– Row-levelTriggers:Onceperrow– Statement-levelTriggers:OnceperSQLstatement


Triggers:ExampleCREATETRIGGERinit_countBEFOREINSERTONStudents/*Event*/

DECLAREcountINTEGER /*Action*/BEGINcount:=0END

CREATETRIGGERincr_countAFTERINSERTONStudent/*Event*/WHEN(new.age<18) /*Condition*/FOREACHROWBEGIN /*Action*/count:=count+1;END


Triggers• Firsttriggerexecutedbefore theactivatingstatement,second

executesafter theactivatingstatement.• Options:

– “BEFORE”– “AFTER”– “INSTEADOF” (onlyvalidonviews)

• Incombinationwith:– “FOREACHROW”- executeoncepermodifiedrecord– (default)- executeonceperactivatingstatement.

Canalsospecifyusing“FOREACHSTATEMENT”• Incombinationwith:

– “INSERT”– “DELETE”– “UPDATE”


Triggers• Referringtovalues

– Old– New– Setofchangedrecord


Triggers:ExampleCREATETRIGGERyoungSailorUpdate

AFTERINSERTONSAILORSREFERENCINGNEWTABLENewSailorsFOREACHSTATEMENT

INSERTINTOYoungSailors(sid,name,age,rating)SELECTsid,name,age,ratingFROMNewSailorsNWHEREN.age<=18


Triggersv/sConstraints• Oftenusedtomaintainconsistency

– Canyouuseaforeignkey?– Foreignkeysarenotdefinedoperationally

• Constraintsareeasiertounderstandthantriggers• Triggersaremorepowerful.

– Oftenusedtofilloutfieldsinaform– Checkcomplexactions(suchascreditlimitinashoppingapplication)

– Checkpreferredcustomerstatus– Generatelogsforauditingandsecuritychecks.– InternallycanbeusedbytheDBMSforreplicationmanagement.


SQL: Queries, Programming, Triggerspages.cs.wisc.edu/~jignesh/cs564/notes/lec09-SQL.pdf · SQL...

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