1

Information Systems

Chapter 6 Database Queries

2

SQL Queries

3

SQL Query

Basic form:

SELECT attributes FROM relations (possibly multiple, joined) WHERE conditions (selections)

SELECT attributes FROM relations (possibly multiple, joined) WHERE conditions (selections)

4

Simple SQL Query

PName Price CategoryManufactu

rer

Gizmo $19.99 GadgetsGizmoWor

ks

Powergizmo

$29.99 GadgetsGizmoWor

ks

SingleTouch

$149.99Photograp

hyCanon

MultiTouch

$203.99 Household Hitachi

SELECT *FROM ProductWHERE category=‘Gadgets’

SELECT *FROM ProductWHERE category=‘Gadgets’

Product

PName Price CategoryManufactu

rer

Gizmo $19.99 GadgetsGizmoWor

ks

Powergizmo

$29.99 GadgetsGizmoWor

ks

“selection”

5

Simple SQL Query

PName Price CategoryManufactu

rer

Gizmo $19.99 GadgetsGizmoWor

ks

Powergizmo

$29.99 GadgetsGizmoWor

ks

SingleTouch

$149.99Photograp

hyCanon

MultiTouch

$203.99 Household Hitachi

SELECT PName, Price, ManufacturerFROM ProductWHERE Price > 100

SELECT PName, Price, ManufacturerFROM ProductWHERE Price > 100

Product

PName PriceManufactu

rer

SingleTouch

$149.99 Canon

MultiTouch

$203.99 Hitachi

“selection” and“projection”

6

A Notation for SQL Queries

SELECT PName, Price, ManufacturerFROM ProductWHERE Price > 100

SELECT PName, Price, ManufacturerFROM ProductWHERE Price > 100

Product(PName, Price, Category, Manfacturer)

Answer(PName, Price, Manfacturer)

Input Schema

Output Schema

7

Selections

What goes in the WHERE clause:• x = y, x < y, x <= y, etc

– For number, they have the usual meanings– For CHAR and VARCHAR: lexicographic

ordering• Expected conversion between CHAR and

VARCHAR

– For dates and times, what you expect...

• Pattern matching on strings: s LIKE p (next)

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The LIKE operator

• s LIKE p: pattern matching on strings

• p may contain two special symbols:– % = any sequence of characters– _ = any single character

Product(Name, Price, Category, Manufacturer)Find all products whose name mentions ‘gizmo’:

SELECT *FROM ProductsWHERE PName LIKE ‘%gizmo%’

SELECT *FROM ProductsWHERE PName LIKE ‘%gizmo%’

9

Eliminating Duplicates

Compare to:

SELECT DISTINCT categoryFROM Product

SELECT DISTINCT categoryFROM Product

Household

Photography

Gadgets

Category

SELECT categoryFROM Product

SELECT categoryFROM Product

Household

Photography

Gadgets

Gadgets

Category

10

Ordering the Results

SELECT pname, price, manufacturerFROM ProductWHERE category=‘gizmo’ AND price > 50ORDER BY price, pname

SELECT pname, price, manufacturerFROM ProductWHERE category=‘gizmo’ AND price > 50ORDER BY price, pname

Ordering is ascending, unless you specify the DESC keyword.

Ties are broken by the second attribute on the ORDER BY list, etc.

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Ordering the Results

SELECT CategoryFROM ProductORDER BY PName

SELECT CategoryFROM ProductORDER BY PName

PName Price CategoryManufactu

rer

Gizmo $19.99 GadgetsGizmoWor

ks

Powergizmo

$29.99 GadgetsGizmoWor

ks

SingleTouch

$149.99Photograp

hyCanon

MultiTouch

$203.99 Household Hitachi

?

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Ordering the Results

SELECT DISTINCT categoryFROM ProductORDER BY category

SELECT DISTINCT categoryFROM ProductORDER BY category

Compare to:

Category

Gadgets

Household

Photography

SELECT DISTINCT categoryFROM ProductORDER BY PName

SELECT DISTINCT categoryFROM ProductORDER BY PName ?

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Joins in SQL

• Connect two or more tables:PName Price Category

Manufacturer

Gizmo $19.99 GadgetsGizmoWork

s

Powergizmo $29.99 GadgetsGizmoWork

s

SingleTouch $149.99Photograph

yCanon

MultiTouch $203.99 Household Hitachi

Product

Company CName StockPrice Country

GizmoWorks 25 USA

Canon 65 Japan

Hitachi 15 Japan

What isthe Connection

betweenthem ?

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Joins

Product (pname, price, category, manufacturer)Company (cname, stockPrice, country)

Find all products under $200 manufactured in Japan;return their names and prices.

SELECT PName, PriceFROM Product, CompanyWHERE Manufacturer=CName AND Country=‘Japan’ AND Price <= 200

SELECT PName, PriceFROM Product, CompanyWHERE Manufacturer=CName AND Country=‘Japan’ AND Price <= 200

Joinbetween Product

and Company

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Joins in SQL

PName Price CategoryManufactur

er

Gizmo $19.99 GadgetsGizmoWork

s

Powergizmo

$29.99 GadgetsGizmoWork

s

SingleTouch

$149.99Photograp

hyCanon

MultiTouch

$203.99 Household Hitachi

Product Company

Cname StockPrice Country

GizmoWorks 25 USA

Canon 65 Japan

Hitachi 15 Japan

PName Price

SingleTouch

$149.99

SELECT PName, PriceFROM Product, CompanyWHERE Manufacturer=CName AND Country=‘Japan’ AND Price <= 200

SELECT PName, PriceFROM Product, CompanyWHERE Manufacturer=CName AND Country=‘Japan’ AND Price <= 200

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Joins

Product (pname, price, category, manufacturer)Company (cname, stockPrice, country)

Find all countries that manufacture some product in the ‘Gadgets’ category.

SELECT CountryFROM Product, CompanyWHERE Manufacturer=CName AND Category=‘Gadgets’

SELECT CountryFROM Product, CompanyWHERE Manufacturer=CName AND Category=‘Gadgets’

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Joins

Product (pname, price, category, manufacturer)Purchase (buyer, seller, store, product)Person(persname, phoneNumber, city)

Find names of people living in Seattle that bought some product in the ‘Gadgets’ category, and the names of the stores they bought such product from

SELECT DISTINCT persname, storeFROM Person, Purchase, ProductWHERE persname=buyer AND product = pname AND city=‘Seattle’ AND category=‘Gadgets’

SELECT DISTINCT persname, storeFROM Person, Purchase, ProductWHERE persname=buyer AND product = pname AND city=‘Seattle’ AND category=‘Gadgets’

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Disambiguating Attributes

• Sometimes two relations have the same attr:Person(pname, address, worksfor)Company(cname, address)

SELECT DISTINCT pname, addressFROM Person, CompanyWHERE worksfor = cname

SELECT DISTINCT pname, addressFROM Person, CompanyWHERE worksfor = cname

SELECT DISTINCT Person.pname, Company.addressFROM Person, CompanyWHERE Person.worksfor = Company.cname

SELECT DISTINCT Person.pname, Company.addressFROM Person, CompanyWHERE Person.worksfor = Company.cname

Whichaddress ?

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Tuple Variables

SELECT DISTINCT x.storeFROM Purchase AS x, Purchase AS yWHERE x.product = y.product AND y.store = ‘BestBuy’

SELECT DISTINCT x.storeFROM Purchase AS x, Purchase AS yWHERE x.product = y.product AND y.store = ‘BestBuy’

Find all stores that sold at least one product that the store‘BestBuy’ also sold:

Answer (store)

Product (pname, price, category, manufacturer)Purchase (buyer, seller, store, product)Person(persname, phoneNumber, city)

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Tuple VariablesGeneral rule: tuple variables introduced automatically by the system: Product ( name, price, category, manufacturer)

Becomes:

Doesn’t work when Product occurs more than once:In that case the user needs to define variables explicitely.

SELECT name FROM Product WHERE price > 100

SELECT name FROM Product WHERE price > 100

SELECT Product.name FROM Product AS Product WHERE Product.price > 100

SELECT Product.name FROM Product AS Product WHERE Product.price > 100

21

Renaming Columns

PName Price CategoryManufactu

rer

Gizmo $19.99 GadgetsGizmoWor

ks

Powergizmo

$29.99 GadgetsGizmoWor

ks

SingleTouch

$149.99Photograp

hyCanon

MultiTouch

$203.99 Household Hitachi

SELECT Pname AS prodName, Price AS askPriceFROM ProductWHERE Price > 100

SELECT Pname AS prodName, Price AS askPriceFROM ProductWHERE Price > 100

Product

prodName askPrice

SingleTouch

$149.99

MultiTouch

$203.99

Query withrenaming