EmilSekerinski,McMasterUniversity,WinterTerm16/17COMPSCI1MD3IntroductiontoProgramming

AnSQLquerygoesintoabar,walksuptotwotablesandasks,"CanIjoinyou?"

Likeprogramminglanguages,databasesallowtoprocessandstoredata.However:¡  dataisstoredindedicatedfilesandloadedasneeded:data

canbemuchlargerthantheavailablememory(retailercatalogue,bankaccounts,personnelrecords);thereisnoneedtoexplicitlyopenfilesandreaddata

¡  datacanbeaccessed"simultaneously"by"clients"¡  dataisatomicallyupdated:dataiseitherstoredornot,but

neverbecomescorrupt,evenincaseoffailure(diskerror,networkdisconnection)

¡  datamustbestructuredinspecificways,e.g.astablesinrelationaldatabases

¡  complexqueriescanbewritteninadedicatedquerylanguage

Dataitemsandtheirrelationshipisstoredintables.Atableisacollectionofrecords(objects,entities)withfields(attributes).

Movie: MovieId Title Genre Rating

101 Casablanca dramaromance PG

102 BacktotheFuture comedyadventure PG

103 Monsters,Inc animationcomedy G

104 FieldofDreams fantasydrama PG

105 Alien sci-fihorror R

106 Unbreakable thriller PG-13

107 X-Men actionsci-fi PG-13

5022 Elisabeth dramaperiod R

5793 LifeofBrian comedy R

7442 12AngryMen drama PG

Adatabaseschemaspecifiesthetypesofthefieldsofeachtable:¡  INTEGER:upto8bytesintegers¡  REAL:8bytefloats¡  TEXT:Unicodestrings¡  BOOLEAN:storedas0and1¡  DATE:storedasnumericvalue

Aprimarykeyisafieldthatuniquelyidentifiesarecord.Thekeyfieldsarespecifiedintheschema.Bydefault,fieldsareTEXT:Movie(MovieIDINTEGERPRIMARYKEY,Title,Genre,Rating)

SQLiteisawidelyusedserverless(databasefilesarestoredlocally)relationaldatabasethatcomespreinstalledorcanbedownloadedatsqlite.orgAllrelationaldatabaseshavesimilarcommands.WeshowtheuseofSQLitefromthecommandlineandwithPython.ForexampleintheMacOSXorLinuxcommandline:$ sqlite3 videostore.dbSQLite version 3.8.5 2014-08-15 22:37:57Enter ".help" for usage hints.sqlite>...sqlite> .quit

create table Table (Column [Type], …)insert into Table values (Expr, …)Allcommandshavetobeterminatedby;create table Movie(MovieId INTEGER PRIMARY KEY, Title, Genre, Rating);

insert into Movie values(101, 'Casablanca', 'drama romance', 'PG');

insert into Movie values(102, 'Back to the Future', 'comedy adventure', 'PG');

insert into Movie values(103, 'Monsters, Inc', 'animation comedy', 'G');

insert into Movie values(101, 'Field of Dreams', 'fantasy drama', 'PG');Error: UNIQUE constraint failed: Movie.MovieId

Supposewewantkeeptrackwhichcustomerrentedwhichvideo:customersandtherentsrelationshipareexpressedastablesCustomer:Rents:

CustomerId Name Address

101 DennisCook 123Broadwalk

102 DougNickle 456ParkPlace

103 RandyWolf 789PacificAvenue

104 AmyYao 321StJamesPlace

105 RobertMwanri 654MarvinGardens

106 DavidCoggin 987CharlesPlace

CustomerId MovieId DateRented DueDate

103 104 3-12-2017 3-13-2017

103 5022 3-28-2017 3-29-2017

105 107 3-28-2017 3-29-2017

create table Customer(CustomerId INTEGER PRIMARY KEY, Name, Address);create table Rents(CustomerId INTEGER, MovieId INTEGER, DateRented DATE, DateDue DATE);

insert into Customer values(101, 'Dennis Cook', '123 Broadwalk');insert into Customer values(102, 'Doug Nickle', '456 Park Place');...

insert into Rents values(103, 104, '3-12-2016', '3-13-2016');insert into Rents values(103, 5022, '3-28-2016', '3-29-2016');insert into Rents values(105, 107, '3-28-2016', '3-29-2016');

update Table set Column = Expr, … [where Expr]delete from Table [where Expr] Forexample:update Movie set Genre = 'thriller drama' where title = 'Unbreakable';

delete from Movie where Rating = 'R';

select Column, … from Table, … where Cond [order by Column, …]Allcolumns(fields)areselectedwith*;theconditionisaBooleanexpressionthatcanhaveconstructsaslike:select * from Movie;select * from Customer;select * from Rents;

select Name, Address from Customer;select * from Movie where Genre like '%comedy%';select * from Movie where Rating = 'PG' order by Title;

select Column, … from Table, … join Table [using(Column)] … where Cond [order by Column, …]Combinedresultofjoiningtables:select * from Customer join Rents using(CustomerId);select * from Customer join Rents using(CustomerId) join Movie using(MovieId);

Allcustomernames,movietitles,andduedates:select Customer.Name, Movie.Title, Rents.DateDue from Customer join Rents using(CustomerId) join Movie using(MovieId);

TitlesofallmoviesofRandyWolfwithduedates:select Movie.Title, Rents.DateDue from Customer join Rents using(CustomerId) join Movie using(MovieId) where Customer.Name = 'Randy Wolf';

Ifacustomerisdeletedwhohasstillvideosrented,thentheRentstablewouldrefertoacustomerthatdoesnotexist,whichisaninconsistencyinthedatabase.Triggerscanautomaticallyensurethiskindofconsistency:create trigger Name delete on Table [when Expr] begin statement; ... endThestatementcanbeanyofinsert,delete,updateandoldcanbeusedtorefertothevaluewhichissupposedtobedeleted,create trigger delete_customer delete on Customer begin delete from Rents where Rents.customerid = old.customerid; end;

delete from Customer where customerid = 103;willnowautomaticallyalsoexecutedelete from Rents where Rents.customerid = 103;Triggerscanalsobecreatedforinsertionandupdatescreate trigger Name insert on Table [when Expr] begin statement; ... endcreate trigger Name update on Table [when Expr] begin statement; ... endThestatementofaninsertionmayusenewtorefertothevaluetobeinserted,thestatementofanupdatemayuseoldandnewtorefertheoldandnewvalue.

SQLisastandardizedlanguagethatwasfirstproposedin1974forCodd'srelationaldatamodelfrom1970.Besidesqueries(select)italsosupportsmanipulation(create,insert,update,delete,…)transactions(begin,commit,rollback),authorization,programmingconstructsSQLismoregeneralthantheexamplessuggest,e.g.theresultofanSQLquerymaybetheinputofanotherone.Alloftoday'srelationaldatabasessupportSQL,withvariations.Morerecently,also"NoSQL"databasesfordifferentlystructureddataandcloudstoragearebecomingpopular.

Programscanstoredatainfileswiththebuild-infunctions;alternatively,programscanstoredatainadatabase.PythonhasalibrarymodulethroughwhichanSQLite3databasecanbeaccessed:import sqlite3db = sqlite3.connect('…/videostore.db')...db.close()Dataisconvertedasfollows:null ↔︎ Noneinteger ↔︎ intreal ↔︎ floattext ↔︎ str

AllSQLstatementsarepassedasstringtothedatabase.AcursorisneededtoiterateovertheresultsofanSQLquery:c = db.cursor()c.execute('select * from Customer order by Name')print(c.fetchone())Ifthereisnoresult,Noneisreturned.SeveralSQLrowscanberetrievedasalistoftuplesc.execute('select * from Customer order by Name')print(c.fetchall()) Acursorcanusedasaniterator:for row in c.execute('select * from Customer'): print(row)

SQLdatamanipulationstatementsdonothaveaneffectrightaway,onlyaftercommit.c.execute("insert into Customer values(103, 'Randy Wolf', '789 Pacific Avenue')")db.commit()

c.execute("delete from Customer where CustomerId = 103")db.commit()SinceconstructingSQLstringsistedious,aspecialformatinsupported,inwhich?inSQLarereplaceddef insert_customer(db, customerid, name, address): c = db.cursor() c.execute("insert into Customer values(?, ?, ?)", \ (customerid, name, address)) db.commit()

insert_customer(db, 103, 'Randy Wolf', \ '789 Pacific Avenue')