1 Information Systems Chapter 6 Database Queries.
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Transcript of 1 Information Systems Chapter 6 Database Queries.
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)
8
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.
11
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
?
12
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 ?
13
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 ?
14
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
15
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’
17
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 ?
19
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