Download Databases - McMaster University

EmilSekerinski,McMasterUniversity,WinterTerm16/17
COMPSCI1MD3IntroductiontoProgramming
AnSQLquerygoesintoabar,walksuptotwotables
andasks,"CanIjoinyou?"
Likeprogramminglanguages,databasesallowtoprocessand
storedata.However:
¡  dataisstoredindedicatedfilesandloadedasneeded:data
canbemuchlargerthantheavailablememory(retailer
catalogue,bankaccounts,personnelrecords);thereisno
needtoexplicitlyopenfilesandreaddata
¡  datacanbeaccessed"simultaneously"by"clients"
¡  dataisatomicallyupdated:dataiseitherstoredornot,but
neverbecomescorrupt,evenincaseoffailure(diskerror,
networkdisconnection)
¡  datamustbestructuredinspecificways,e.g.astablesin
relationaldatabases
¡  complexqueriescanbewritteninadedicatedquery
language
Dataitemsandtheirrelationshipisstoredintables.Atableisa
collectionofrecords(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.Thekey
fieldsarespecifiedintheschema.Bydefault,fieldsareTEXT:
Movie(MovieIDINTEGERPRIMARYKEY,Title,Genre,Rating)
SQLiteisawidelyusedserverless(databasefilesarestored
locally)relationaldatabasethatcomespreinstalledorcanbe
downloadedatsqlite.org
Allrelationaldatabaseshavesimilarcommands.Weshowthe
useofSQLitefromthecommandlineandwithPython.For
exampleintheMacOSXorLinuxcommandline:
$ sqlite3 videostore.db
SQLite version 3.8.5 2014-08-15 22:37:57
Enter ".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:
customersandtherentsrelationshipareexpressedastables
CustomerId Name
Address
Customer:
101
DennisCook
123Broadwalk
102
DougNickle
456ParkPlace
103
RandyWolf
789PacificAvenue
104
AmyYao
321StJamesPlace
105
RobertMwanri
654MarvinGardens
106
DavidCoggin
987CharlesPlace
CustomerId MovieId
DateRented DueDate
Rents:
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*;theconditionisaBooleanexpression
thatcanhaveconstructsaslike:
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,thentheRentstablewould
refertoacustomerthatdoesnotexist,whichisaninconsistencyinthe
database.Triggerscanautomaticallyensurethiskindofconsistency:
create trigger Name delete on Table [when Expr]
begin statement; ... end
Thestatementcanbeanyofinsert,delete,updateandoldcanbe
usedtorefertothevaluewhichissupposedtobedeleted,
create trigger delete_customer delete on Customer
begin
delete from Rents where Rents.customerid =
old.customerid;
end;
delete from Customer where customerid = 103;
willnowautomaticallyalsoexecute
delete from Rents where Rents.customerid = 103;
Triggerscanalsobecreatedforinsertionandupdates
create trigger Name insert on Table [when Expr]
begin statement; ... end
create trigger Name update on Table [when Expr]
begin statement; ... end
Thestatementofaninsertionmayusenewtorefertothevaluetobe
inserted,thestatementofanupdatemayuseoldandnewtorefertheold
andnewvalue.
SQLisastandardizedlanguagethatwasfirstproposedin1974
forCodd'srelationaldatamodelfrom1970.Besidesqueries
(select)italsosupportsmanipulation(create,insert,
update,delete,…)transactions(begin,commit,
rollback),authorization,programmingconstructs
SQLismoregeneralthantheexamplessuggest,e.g.theresult
ofanSQLquerymaybetheinputofanotherone.
Alloftoday'srelationaldatabasessupportSQL,withvariations.
Morerecently,also"NoSQL"databasesfordifferently
structureddataandcloudstoragearebecomingpopular.
Programscanstoredatainfileswiththebuild-infunctions;
alternatively,programscanstoredatainadatabase.Pythonhas
alibrarymodulethroughwhichanSQLite3databasecanbe
accessed:
import sqlite3
db = sqlite3.connect('…/videostore.db')
...
db.close()
Dataisconvertedasfollows:
null
↔︎
None
integer ↔︎
int
real
↔︎
float
text
↔︎
str
AllSQLstatementsarepassedasstringtothedatabase.Acursoris
neededtoiterateovertheresultsofanSQLquery:
c = db.cursor()
c.execute('select * from Customer order by Name')
print(c.fetchone())
Ifthereisnoresult,Noneisreturned.SeveralSQLrowscanbe
retrievedasalistoftuples
c.execute('select * from Customer order by Name')
print(c.fetchall())
Acursorcanusedasaniterator:
for row in c.execute('select * from Customer'):
print(row)
SQLdatamanipulationstatementsdonothaveaneffectrightaway,onlyafter
commit.
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?
inSQLarereplaced
def 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')