cmpt354: databasesystemi · sql provides pattern matching support with the like operator and two...

CMPT 354:Database System I

Lecture 3. SQL Basics

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Announcements!

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SQLMotivation• Darktimesin2000s• Arerelationaldatabasesdead?

• Now,asbefore:everyonesellsSQL• Pig,Hive,Impala• SparkSQL

• NoSQL• “NonSQL”• “Not-Only-SQL”• “Not-Yet-SQL”

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SQL:Introduction

• “S.Q.L.”or“sequel”• Supportedbyallmajorcommercialdatabasesystems• Standardized– manynewfeaturesovertime• Declarativelanguage

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SQLisa…

• DataDefinitionLanguage(DDL)• Definerelationalschema• Create/alter/deletetablesandtheirattributes

• DataManipulationLanguage(DML)• Insert/delete/modifytuplesintables• Queryoneormoretables– discussednext!

Outline

• Single-table Queries• The SFW query• Usefuloperators:DISTINCT,ORDER BY,LIKE• Handlemissingvalues:NULLs

• Multiple-table Queries• Foreign key constraints• Joins: basics• Joins: SQL semantics

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• To write the query, ask yourself three questions:• Which table are you interested in?• Which rows are you interested in?• Which columns are you interested in?

The SFWQuery

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SELECT <columns>FROM <table name>WHERE <conditions>

Conditions

• Which rows are you interested in?• WHERE gpa > 3.5• WHERE school = ‘SFU’ AND gpa > 3.5• WHERE (school = ‘SFU’ OR school = ‘UBC’) AND gpa > 3.5• WHERE age *365 > 7500

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Columns

• Which columns are you interested in?• SELECT *• SELECT name, age• SELECT name as studentName, age• SELECT name, age *365 as ageDay

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AFewDetails

• SQLcommands arecaseinsensitive:• Same:SELECT,Select,select• Same:Student,student• Same:gpa, GPA

• Values arenot:• Different: 'SFU','sfu'

• SQLstringsareenclosedinsinglequotes• e.g.name='Mike’• Singlequotesinastringcanbespecifiedusinganinitialsinglequotecharacterasanescape• author='ShaqO''Neal'

• Stringscanbecomparedalphabetically withthecomparisonoperators• e.g.'fodder'<'foo'isTRUE

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DISTINCT:EliminatingDuplicates

SELECT DISTINCT SchoolFROM Students

Versus

SELECT SchoolFROM Students

School

SFU

UBC

UT

School

SFU

SFU

UBC

UT

UT

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ORDERBY:SortingtheResults

SELECT name, gpa, ageFROM StudentsWHERE school = 'SFU'ORDER BY gpa DESC, age ASC

• TheoutputofanSQLquerycanbeordered• Byanynumberofattributes,and• Ineitherascendingordescendingorder

• Thedefaultistouseascendingorder,thekeywordsASCandDESC,followingthecolumnname,setstheorder

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LIKE:SimpleStringPatternMatching

SELECT *FROM StudentsWHERE name LIKE ' Sm_t%'

SQLprovidespatternmatchingsupportwiththeLIKEoperatorandtwosymbols• The% symbolstandsforzeroormorearbitrarycharacters• The_ symbolstandsforexactlyonearbitrarycharacter• The% and _ characterscanbeescapedwith\

• E.g., nameLIKE ’Michael\_Jordan'

Exercise - 1

• Which names will be returned?

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SELECT *FROM StudentsWHERE name LIKE 'Sm_t%'

1. Smit2. SMIT3. Smart4. Smith5. Smythe6. Smut7. Smeath8. Smt

Exercise - 1

• Which names will be returned?

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SELECT *FROM StudentsWHERE name LIKE 'Sm_t%'

1. Smit2. SMIT3. Smart4. Smith5. Smythe6. Smut7. Smeath8. Smt

1, 4, 5, 6

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NULLSinSQL

• Wheneverwedon’thaveavalue,wecanputaNULL• Canmeanmanythings:

• Valuedoesnotexists• Valueexistsbutisunknown• Valuenotapplicable• Etc.

• NULLconstraints

CREATE TABLE Students (name CHAR(20) NOT NULL,age CHAR(20) NOT NULL,gpa FLOAT

)

What will happen?

name age gpa

Mike 20 4.0

Joe 18 NULL

Alice 21 3.8

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1. SELECT gpa*100 FROM students2. SELECT name FROM students WHERE gpa > 3.53. SELECT name FROM students WHERE age > 15 OR gpa > 3.5

Two Important Rules

• Arithmeticoperations (+, -, *, /) onnullsreturnNULL• NULL *100

• NULL• NULL*0

• NULL

• ComparisonswithnullsevaluatetoUNKNOWN• NULL > 3.5

• UNKNOWN• NULL=NULL

• UNKNOWN

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1. SELECT gpa*100 FROM students

2. SELECT gpa*0 FROM students

3. SELECT name FROM students WHERE gpa > 3.5

4. SELECT name FROM students WHERE gpa = NULL

Combinations of true, false, unknown

• Truthvaluesforunknown results• true OR unknown =true,• falseOR unknown= unknown,• unknown OR unknown =unknown ,• true AND unknown =unknown,• false AND unknown =false,• unknown AND unknown =unknown

• TheresultofaWHERE clauseistreatedasfalse ifitevaluatestounknown• WHERE unknownà false

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SELECT *FROM students WHEREage > 15 OR gpa > 3.5

SELECT *FROM students WHEREage > 15 AND gpa > 3.5

What will happen?

name age gpa

Mike 20 4.0

Joe 18 NULL

Alice 21 3.8

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1. SELECT gpa*100 FROM students2. SELECT name FROM students WHERE gpa > 3.53. SELECT name FROM students WHERE age > 15 OR gpa > 3.5

gpa

400

NULL

380

name

Mike

Alice

name

Mike

Joe

Alice

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Exercise - 2

• Will it return all students?

SELECT *FROM StudentsWHERE age < 25 OR age >= 25

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Exercise - 2

• Will it return all students?

SELECT *FROM StudentsWHERE age < 25 OR age >= 25

OR age is NULL

Therearespecialoperatorstotestfornullvalues• ISNULLtestsforthepresenceofnullsand• ISNOTNULL testsfortheabsenceofnulls

Outline

• Single-table Queries• The SFW query• Other useful operators: DISTINCT, LIKE, ORDER BY• NULLs

• Multiple-table Queries• Foreign key constraints• Joins: basics• Joins: SQL semantics

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• Foreign-keyconstraint:• student_id referencessid

ForeignKeyconstraints

sid name gpa101 Bob 3.2123 Mary 3.8

student_id cid grade123 354 A123 454 A+156 354 A

Students Enrolled

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• Foreign-keyconstraint:• student_id referencessid

ForeignKeyconstraints

sid name gpa101 Bob 3.2123 Mary 3.8156 Mike 3.7

Students Enrolledstudent_id cid grade

123 354 A123 454 A+156 354 A

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DeclaringForeignKeys

CREATE TABLE Enrolled(student_id CHAR(20),cid CHAR(20),grade CHAR(10),PRIMARY KEY (student_id, cid),FOREIGN KEY (student_id) REFERENCES Students(sid)

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Insertoperations• WhatifweinsertatupleintoEnrolled,butnocorrespondingstudent?• INSERTisrejected

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sid name gpa123 Mary 3.8156 Mike 3.7


123 354 A123 454 A+156 354 A190 354 A

Delete operations• Whatifwedeleteastudent,whohasenrolledcourses?• Disallowthedelete(ONDELETERESTRICT)

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sid name gpa123 Mary 3.8


123 354 A123 454 A+156 354 A

156 Mike 3.7

ONDELETERESTRICT

CREATE TABLE Enrolled(student_id CHAR(20),cid CHAR(20),grade CHAR(10),PRIMARY KEY (student_id, cid),FOREIGN KEY (student_id) REFERENCES Students(sid)ON DELETE RESTRICT

)


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Delete operations• Whatifwedeleteastudent,whohasenrolledcourses?• Removeallofthecoursesforthatstudent(ONDELETE

CASCADE)

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123 354 A123 454 A+156 Mike 3.7156 354 A

ONDELETECASCADE

CREATE TABLE Enrolled(student_id CHAR(20),cid CHAR(20),grade CHAR(10),PRIMARY KEY (student_id, cid),FOREIGN KEY (student_id) REFERENCES Students(sid)ON DELETE CASCADE

)


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Delete operations• Whatifwedeleteastudent,whohasenrolledcourses?• Set Foreign Key to NULL (ONDELETESETNULL)

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123 354 A123 454 A+156 354 A

156 Mike 3.7NULL 354 A

Interestingly, although it satisfies the foreign-key constraint,it violates the primary-key constraint, thus the deletionoperation is disallowed.

ONDELETESETNULL

CREATE TABLE Enrolled(student_id CHAR(20),cid CHAR(20),grade CHAR(10),PRIMARY KEY (student_id, cid),FOREIGN KEY (student_id) REFERENCES Students(sid)ON DELETE SET NULL

)


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Outline


• Multiple-table Queries• Foreign key constraints• Joins: basics• Joins: SQL semantics• SetOperators

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Why do we havemultiple tables?

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123 354 A123 454 A+156 Mike 3.7156 354 A

EnrolledStudents

student_id name gpa cid grade123 Mary 3.8 354 A123 Mary 3.8 454 A+156 Mike 3.7 354 A

VS.

• Multiple tables• Data updating is easier (e.g., update Mary’s gpa to 3.9)• Queryingeachindividualtableis faster (e.g., retrieveMary’s gpa)

• Asingle table• Data exchange is easier (e.g., share your data withothers)• Avoid the cost of joining multiple tables (e.g., retrievalall the courses that Mary has taken)

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Storedataintomultiple tablesvs.singletable

Joins

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SELECT <columns>FROM <table names>WHERE <conditions>

TheSFWqueryoverasingletable

TheSFWqueryovermultipletables

Whichrowsareyouinterestedin?

Whichrowsareyouinterestedin?

Howtojointhemultipletables?

Joins:Example

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SELECT name

FROM Students, Enrolled

WHERE sid = student_id AND

cid = 354 AND grad = ‘A+’Whichrowsareyouinterestedin?

Howtojointhetwotables?



123 354 A+123 454 A+156 Mike 3.7156 354 A

FindallstudentwhohavegotanA+in354;returntheirnamesandgpas

Otherwaystowritejoins

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SELECT nameFROM Students, EnrolledWHERE sid = student_id AND

cid = 354 AND grad = ‘A+’

SELECT nameFROM StudentsJOIN Enrolled ON sid = student_id

AND cid = 354 AND grad = ‘A+’

SELECT nameFROM StudentsJOIN Enrolled ON sid = student_idWHERE cid = 354 AND grad = ‘A+’

The Need fo TupleVariable

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Students Enrolledstudent_id cid name grade123 354 DBI A+123 454 DBII A+156 354 DBI A

156 Mike 3.7

SELECT name

FROM Students, Enrolled

WHERE sid = student_id

Whichname?

TupleVariable

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Students Enrolledstudent_id cid name grade123 354 DBI A+123 454 DBII A+156 354 DBI A

156 Mike 3.7

SELECT Students.nameFROM Students, EnrolledWHERE sid = student_id

SELECT S.nameFROM Students S, EnrolledWHERE sid = student_id

Outline



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Meaning(Semantics)ofJoinQueries

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SELECT x1.a1, x1.a2, …, xn.akFROM R1 AS x1, R2 AS x2, …, Rn AS xnWHERE Conditions(x1,…, xn)

Answer={}for x1 in R1 do

for x2 in R2 do…..

for xn in Rn doif Conditions(x1,…,xn)

then Answer=AnswerÈ {(x1.a1,x1.a2,…,xn.ak)}return Answer

Thisiscallednestedloopsemantics sinceweareinterpretingwhatajoinmeansusinganestedloop

Note:this isamultisetunion

Threesteps

1. Takecrossproduct• R1 × R2 × … × Rn

2. Applyconditions• Conditions(x1,…, xn)

3. Applyprojections• x1.a1, x1.a2, …, xn.ak

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SELECT x1.a1, x1.a2, …, xn.akFROM R1 AS x1, R2 AS x2, …, Rn AS xnWHERE Conditions(x1,…, xn)

Note:ThisisNOThowtheDBMSexecutesthequery.

Exercise

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SELECT nameFROM Students, EnrolledWHERE sid = student_id AND grade >= ‘A’



123 354 A+123 454 A+156 Mike 3.7156 354 A

nameMary

nameMaryMike

nameMaryMikeMary

nameMaryMaryMike

Whichone(s)arecorrect?

(A) (B) (C) (D)

Outline



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SetOperations

• SQLsupportsunion,intersectionandsetdifferenceoperations• CalledUNION,INTERSECT,andEXCEPT• Theseoperationsmustbeperformedonunioncompatibletables

• AlthoughtheseoperationsaresupportedintheSQLstandard,implementationsmayvary• EXCEPTmaynotbeimplemented

• Whenitis,itissometimescalledMINUS

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OneofTwoCourses

• Findallstudentswhohavetakeneither354or454

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123 354 A+123 454 A+156 Mike 3.7156 354 A

SELECT nameFROM Students, EnrolledWHERE sid = student_id AND (cid = 354 OR cid = 454)

OneofTwoCourses- UNION

• Findallstudentswhohavetakeneither354or454

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123 354 A+123 454 A+156 Mike 3.7156 354 A

SELECT nameFROM Students, EnrolledWHERE sid = student_id AND cid = 354UNIONSELECT nameFROM Students, EnrolledWHERE sid = student_id AND cid = 454

BothCourses

• Findallstudentswhohavetakenboth354and454

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123 354 A+123 454 A+156 Mike 3.7156 354 A

SELECT nameFROM Students S, Enrolled EWHERE sid = student_id AND

(E.cid = 354 AND E.cid = 454)

BothCoursesAgain


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123 354 A+123 454 A+156 Mike 3.7156 354 A

SELECT nameFROM Students S, Enrolled E1, Enrolled E2WHERE S.sid = E1.student_id AND S.sid = E2.student_id

AND (E1.cid = 354 AND E2.cid = 454)

BothCourses- INTERSECT


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123 354 A+123 454 A+156 Mike 3.7156 354 A

SELECT nameFROM Students, Enrolled WHERE sid = student_id AND cid = 354INTERSECTSELECT nameFROM Students, EnrolledWHERE sid = student_id AND cid = 454

OneCourseButNotTheOther

• Findallstudentswhohavetaken354butnot454

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123 354 A+123 454 A+156 Mike 3.7156 354 A

SELECT nameFROM Students, Enrolled WHERE sid = student_id AND cid = 354EXCEPTSELECT nameFROM Students, EnrolledWHERE sid = student_id AND cid = 454

SetOperationsandDuplicates

• UnlikeotherSQLoperations,UNION,INTERSECT,andEXCEPT querieseliminateduplicatesbydefault• SQLallowsduplicatestoberetainedinthesethreeoperationsusingtheALL keyword(i.e.,multi-setoperations)

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SELECT nameFROM Students, Enrolled WHERE sid = student_id AND cid = 354INTERSECT ALLSELECT nameFROM Students, EnrolledWHERE sid = student_id AND cid = 454

Acknowledge

• Some lecture slides were copied from or inspired by thefollowing course materials• “W4111: Introduction to databases” by Eugene Wu atColumbia University• “CSE344: IntroductiontoDataManagement” by Dan Suciu atUniversityof Washington• “CMPT354: Database System I” by JohnEdgar at Simon FraserUniversity• “CS186: Introduction to Database Systems” by Joe Hellersteinat UC Berkeley• “CS145: IntroductiontoDatabases” by Peter Bailis at Stanford• “CS348:IntroductiontoDatabaseManagement” by GrantWeddell at University of Waterloo

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cmpt354: databasesystemi · sql provides pattern matching support with the like operator and two...

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