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Relational Databases
The Relational
Model
Jennifer Widom
The Relational Model
 Used by all major commercial database systems
 Very simple model
 Query with high-level languages: simple yet expressive
 Efficient implementations
Jennifer Widom
The Relational Model
Schema = structural description of relations in database
Instance = actual contents at given point in time
Jennifer Widom
The Relational Model
Database
set of named
relationsof(or
tables) in database
Schema = structural
description
relations
Each
relation
has acontents
set of named
attributes
columns)
Instance
= actual
at given
point in(or
time
Each tuple (or row) has a value for each attribute
Each attribute has a type (or domain)
Jennifer Widom
The Relational Model
Database
set of named
relations
(or
tables)inindatabase
Schema– =structural
structural
description
relations
database
Schema
description
ofofrelations
Each
relation
has acontents
set of named
attributes
(or
columns)
Instance
actual
contents
given
pointinintime
time
Instance
–=actual
atatgiven
point
Each tuple (or row) has a value for each attribute
Each attribute has a type (or domain)
Jennifer Widom
The Relational Model
Database
set value
of named
relations
(or
Schema
structural
description
relations
database
Schema
NULL
– special
– =structural
description
for
“unknown”
ofofrelations
ortables)
“undefined”
inindatabase
Each
relation
has acontents
set of named
attributes
(or
columns)
Instance
actual
contents
given
pointinintime
time
Instance
–=actual
atatgiven
point
Each tuple (or row) has a value for each attribute
Each attribute has a type (or domain)
Jennifer Widom
The Relational Model
Database
set value
of
named
relations
(or
tables)
Schema
structural
description
relations
database
Schema
NULL
Key
– –attribute
special
– =structural
whose
description
for
value
“unknown”
is unique
ofofrelations
orin
“undefined”
eachinin
tuple
database
Each
has acontents
set
of named
attributes
(or
columns)
Instance
=actual
actual
contents
given
point
time
Instance
Or
setrelation
of –attributes
whose
combined
atatgiven
point
values
inintime
are
unique
Each tuple (or row) has a value for each attribute
Each attribute has a type (or domain)
Jennifer Widom
The Relational Model
Creating relations (tables) in SQL
Create Table Student(ID, name, GPA, photo)
Create Table College
(name string, state char(2), enrollment integer)
Jennifer Widom
The Relational Model
 Used by all major commercial database systems
 Very simple model
 Query with high-level languages: simple yet expressive
 Efficient implementations
Jennifer Widom