SQL:DataDefinitionLanguageCSC343Winter2018MICHAEL L IUT(MICHAEL.L IUT@UTORONTO.CA)DEPARTMENT OF MATH EMAT ICA L ANDCOMPUTAT IONA L SC IENCE SUN IV E RS IT Y OF TORONTO M ISS ISSAUGA

DatabaseSchemasinSQLSQLisprimarilyaquerylanguage,forgetting informationfromadatabase.◦ DML:DataManipulation Language

ButSQLalso includes adata-definitioncomponent fordescribing databaseschemas.◦ DDL:DataDefinition Language

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Creating(Declaring)aRelationSimply:◦ CREATE TABLE <table_name> (

<list ofelements>);

Deleting arelation:◦ DROPTABLE <table_name>;

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ElementsofTableDeclarationsMostbasicelement: anattribute anditstype.CommonTypes:◦ INTorINTEGER (synonyms).◦ Also:SMALLINT,MEDIUMINT,andBIGINT

◦ REALorFLOATorDOUBLE(synonyms).◦ CHAR(n)=fixed-length stringofncharacters.◦ VARCHAR(n)=variable-length stringofuptoncharacters.

Additional datatypefoundhere:https://dev.mysql.com/doc/refman/5.7/en/data-types.html

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Example:CreateTable

CREATE TABLE Sells (

bar CHAR(20),

beer VARCHAR(20),

price REAL

);

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SQLValues

Integersandrealsarerepresented asyouwouldexpect.

Stringsaretoo,excepttheyrequiresingle quotes.◦ Twosinglequotes =realquote

e.g.‘Joe’’s Bar’.

AnyvaluecanbeNULL◦ Unless attribute hasNOTNULLconstraint

e.g.priceREALnotnull,

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DatesandTimesDATEandTIMEaretypes inSQL◦ MySQLevenhasDATETIME

◦ Rangingfrom1000-01-0100:00:00 to9999-12-3123:59:59

Theformofadatevalue is:DATE‘yyyy-mm-dd’

e.g.DATE‘2007-09-30’isSeptember 30th,2007.

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TimesandValues

Theformofatimevalue is:◦ TIME‘hh:mm:ss’– withanoptionaldecimalpointforafractionofasecond.

e.g.TIME‘15:30:02.5’=twoandahalfseconds after3:30pm.

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DeclaringKeys

Anattributeorlistofattributesmaybedeclared PRIMARYKEYorUNIQUE.

Eithersaysthatnotwotuples oftherelationmayagree inalltheattribute(s) onthe list.

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Example

Beers(name,manf)Bars(name,addr,license)Drinkers(name,addr,phone)Likes(drinker,beer)Sells(bar,beer,price)Frequents(drinker,bar)

Underline=key (tuplescannothavethesamevalueinallkeyattributes)

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DeclaringSingle-AttributeKeys

PlacePRIMARYKEYorUNIQUEKEYafterthetype inthedeclaration oftheattribute.

Example:

CREATE TABLE Beers (

nameCHAR(20)UNIQUE,

manf CHAR(20)

);

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DeclaringMulti-AttributeKeysAkeydeclaration canalsobeanotherelement inthe listofelements ofaCREATE TABLE statement.

Thisformisessential ifthekeyconsists ofmorethanoneattribute.◦ Couldbeusedforone-attributekeys.

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Example

ThebarandbeertogetherarethekeyforSells:

CREATE TABLE Sells (

bar CHAR(20),beer VARCHAR(20),price REAL,PRIMARYKEY(bar,beer)

);

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PrimaryKeyvs.Unique

1. TherecanbeonlyonePRIMARYKEYforarelation,butseveralUNIQUEattributes.

2. NoattributeofaPRIMARYKEYcaneverbeNULLinanytuple.Butattributes declaredUNIQUEmayhaveNULL’s,andtheremaybeseveraltupleswithNULL.

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TypesofConstraints1. Keys

2. Foreign-key (referential-integrity) constraint

3. Domain constraint◦ Constrainvaluesofaparticularattribute.

4. Tuple-based constraint◦ Relationshipamongcomponents.

5. Assertions:anySQLBooleanexpressions

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ForeignKeys

Valuesappearing inattributes ofonerelationmustappeartogether incertainattributes ofanotherrelation.

Example: inSells(bar,beer,price),wemightexpectthatabeervaluealsoappears inBeers.name

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ExpressingForeignKeys

UsekeywordREFERENCES, either:1. Afteranattribute(forone-attribute keys).2. Asanelement oftheschema:

FOREIGNKEY(<listofattributes>)

REFERENCES <relation> (<attributes>)

Referencedattributesmustbedeclared PRIMARYKEYorUNIQUE.

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Example:withAttribute

CREATE TABLE Beers(name CHAR(20) PRIMARYKEY,

manf CHAR(20)

);

CREATE TABLE Sells (bar CHAR(20),beer CHAR(20) REFERENCES Beers(name),

price REAL

);

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Example:AsSchemaElementCREATE TABLE Beers(

name CHAR(20) PRIMARYKEY,

manf CHAR(20)

);

CREATE TABLE Sells (bar CHAR(20),beer CHAR(20),

price REAL

FOREIGNKEY(beer) REFERENCES Beers(name)

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EnforcingForeign-KeyConstraints

Ifthere isaforeign-keyconstraintfromrelationRtorelationS,twoviolations arepossible:

1. AninsertorupdatetoRintroducesvaluesnotfoundinS.

2. Adeletion orupdatetoScausessome tuplesofRto“dangle.”

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ActionsTaken

Example: suppose R=Sells, S=Beers.

AninsertorupdatetoSells that introducesanonexistent beermustberejected.

Adeletion orupdatetoBeersthatremovesabeervaluefoundinsometuples ofSells canbehandled inthreeways.

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ActionsTaken

Default:Rejectthemodification.

Cascade:Makethesamechanges inSells.◦ Deleted beer:delete Sellstuple.◦ Updatedbeer:changevalueinSells.

SetNull:ChangethebeettoNULL.

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Example:Cascade

Delete theBudtuplefromBeers:◦ Thendelete alltuples fromSells thathavebeer=‘Bud’.

UpdatetheBudtuplebychanging ‘Bud’to‘Budweiser’◦ ThenchangeallSellstupleswithbeer=‘Bud’tobeer=‘Budweiser’

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Example:SetNULL

Delete theBudtuplefromBeers:◦ Changealltuples ofSells thathavebeer=‘Bud’tohavebeer=NULL.

UpdatetheBudtuplebychanging ‘Bud’to‘Budweiser’:◦ Samechangeasfordeletion.

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ChoosingaPolicy

Whenwedeclareaforeignkey,wemaychoosepolicies SETNULLorCASCADEindependently fordeletions andupdates.

Followtheforeign-keydeclarationby:ON[UPDATE,DELETE][SETNULLCASCADE]

Twosuchclausesmaybeused.

Otherwise, thedefault (reject)isused.

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Example:SettingPolicyCREATE TABLE Sells (

barCHAR(20),beer CHAR(20),price REAL,FOREIGNKEY(beer)

REFERENCES Beers(name)ONDELETESETNULLONUPDATECASCADE

);

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Attribute-BasedChecks

Constraints onthevalueofaparticular attribute.

AddCHECK(<condition>) tothedeclarationfortheattribute.

Theconditionmayusethenameoftheattribute,butanyotherrelationorattributemustbeinasub-query.

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Example:Attribute-BasedCheck

CREATE TABLE Sells (

barCHAR(20),

beer CHAR(20)CHECK(

beer IN(SELECTnameFROMBeers)),

price REALCHECK(price<=5.00)

);

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TimingofChecks

Attribute-based checksareperformedonlywhenavalueforthatattributeisinserted orupdated.

◦ Example: CHECK(price<=5.00)checkseverynewpriceandrejectsthemodification (forthattuple) ifthepriceismorethan$5.

◦ Example: CHECK(beer IN(SELECTnameFROMBeers))notchecked ifabeer isdeleted fromBeers (unlikeforeign-keys).

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Tuple-BasedChecks

CHECK(<condition>) maybeaddedasarelation-schema element.

Theconditionmayrefertoanyattributeoftherelation.◦ Butotherattributes orrelations requireasub-query.

Checked oninsert orupdateonly.

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Example:Tuple-BasedCheck

• OnlyJoe’sBarcansellbeerformorethan$5.

CREATE TABLE Sells (

bar CHAR(20),

beer CHAR(20),

price REAL,

CHECK(bar=‘Joe’’s Bar’ORprice<=5.00)

);

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Questions?

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I’LL BE ANSWERING QUESTIONS NOW

Q A&

THANKS FOR LISTENING

Citations,ImagesandResourcesDatabaseManagementSystems(3rd Ed.),Ramakrishnan&Gehrke

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

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