Garcia-Molina, Ullman, and Widom
Database Systems
Chapter 2
Data Models
• A data model is a notation for describing the
structure of the data in a database ,
– along with the constraints on that data.
• a day of the week is an integer between 1 and 7
• The data model also normally provides a notation for
describing operations on that data :
– queries and data modifications.
Relational Model
• Relations are tables representing information .
– The set of tuples for a given relation is an instance of that
relation
• Columns are headed by attributes ;
– each attribute has an associated domain , or data type.
• Rows are called tuples , and
– a tuple has one component for each attribute of the
relation.
Schemas
• A relation name , together with the attributes of that
relation and their types , form the relation schema.
– Movies(title, year, length, genre)
• A collection of relation schemas forms a database
schema.
• Particular data for a relation or collection of relations
is called an instance of that relation schema or
database schema.
Keys
• An important type of constraint on relations is the
assertion that an attribute or set of attributes forms
a key for the relation.
– known as key constraints
• No two tuples of a relation can agree on all attributes
of the key ,
– although they can agree on some of the key attributes.
Semistructured Data Model
• In this model , data is organized in a tree or graph
structure .
• XML is an important example of a semistructured
data model
– Tags similar to those used in HTML define the role played
by different pieces of data
• much as the column headers do in the relational model
SQL
• The language SQL is the principal query language for
relational database systems .
• The current standard is called SQL-99 .
• Commercial systems generally vary from this
standard but adhere to much of it .
Relation implementation
• Stored relations, called tables, exist in the database and
can be modified by changing its tuples, as well as
queried.
• Views are relations defined by a computation
– these relations are not stored, but are constructed, in whole or
in part, when needed
• Temporary tables are constructed by SQL during the
execution of queries and data modification
– after the transaction these are thrown away
Data Definition
• SQL has statements to declare elements of a
database schema .
• The CREATE TABLE statement allows us to declare the
schema for stored relations
– specifying the attributes , their types , default values , and
keys.
Altering Relation Schemas
• We can change parts of the database schema with an
ALTER statement .
• These changes include
– adding and removing attributes from relation schemas and
– changing the default value associated with an attribute .
• We may also use a DROP statement to completely
eliminate relations or other schema elements.
Modifying relations examples
DROP TABLE R;
Relation R is no longer part of the database schema, and its tuples can no
longer be accessed
ALTER TABLE MovieStar ADD phone CHAR(16);
ALTER TABLE MovieStar DROP birthdate;
CREATE TABLE MovieStar (
name CHAR(30),
address VARCHAR(255),
gender CHAR(1),
birthdate DATE,
PRIMARY KEY (name)
);
Algebra
• An algebra consists of operators and atomic
operands.
• In relational algebra, the atomic operands are
– Variables that stand for relations
– Constants which are finite relations
Relational Algebra
• This algebra underlies most query languages
for the relational model.
• Its principal operators are
– union , intersection, difference , selection ,
– projection , Cartesian product ,
– natural join , theta-join , and
– renaming .
Section 2.4 Relational algebra and SQL
• From the text
– Relational algebra is not used today as a query language in
commercial DBMS’s, although some of the early
prototypes did use this algebra directly.
– SQL incorporates relational algebra at its center, and many
SQL programs are really “syntactically sugared”
expressions of relational algebra.
Some SQL
CREATE TABLE Product (
maker CHAR(30),
model CHAR(10) PRIMARY KEY,
type CHAR(15)
);
CREATE TABLE PC (
model CHAR(30),
speed DECIMAL(4,2),
ram INTEGER,
hd INTEGER,
price DECIMAL(7,2)
);
CREATE TABLE Laptop (
model CHAR(30),
speed DECIMAL(4,2),
ram INTEGER,
hd INTEGER,
screen DECIMAL(3,1),
price DECIMAL(7,2)
);
CREATE TABLE Printer (
model CHAR(30),
color BOOLEAN,
type CHAR (10),
price DECIMAL(7,2)
);
Selection and Projection
• The selection operator produces a result
consisting of all tuples of the argument
relation that satisfy the selection condition .
• Projection removes undesired columns from
the argument relation to produce the result
Some Data for Product
M
a
k
e
r
M
o
d
e
l
T
y
p
e
A
A
A
A
A
A
B
B
B
B
C
D
D
1001
1002
1003
2004
2005
2006
1004
1005
1006
2007
1007
1008
1009
pc
pc
pc
laptop
laptop
laptop
pc
pc
pc
laptop
pc
pc
pc
D
D
D
E
E
E
E
E
E
E
E
E
F
F
G
H
H
1010
3004
3005
1011
1012
1013
2001
2002
2003
3001
3002
3003
2008
2009
2010
3006
3007
pc
printer
printer
pc
pc
pc
laptop
laptop
laptop
printer
printer
printer
laptop
laptop
laptop
printer
printer
Some data for PC
model
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
speed
2.66
2.10
1.42
2.80
3.20
3.20
2.20
2.20
2.00
2.80
1.86
2.80
3.06
ram
1024
512
512
1024
512
1024
1024
2048
1024
2048
2048
1024
512
hd
250
250
80
250
250
320
200
250
250
300
160
160
80
price
2114
995
478
649
630
1049
510
770
650
770
959
649
529
What PC models have a speed of at least 3.00?
R1 := σspeed ≥ 3.00 (PC)
R2 := πmodel(R1)
model
1005
1006
1013
Some data for Laptop
model
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
speed
2.00
1.73
1.80
2.00
2.16
2.00
1.83
1.60
1.60
2.00
ram
2048
1024
512
512
1024
2048
1024
1024
512
2048
hd
240
80
60
60
120
80
120
100
80
160
screen
20.1
17.0
15.4
13.3
17.0
15.4
13.3
15.4
14.1
15.4
price
3673
949
549
1150
2500
1700
1429
900
680
2300
Which manufacturers make laptops with a hard disk of at least 100GB?
R1 := σhd ≥ 100 (Laptop)
R2 := Product (R1)
R3 := πmaker (R2)
maker
E
A
B
F
G
Joins
• We join two relations by comparing tuples , one from
each relation.
• In a natural join , we splice together those pairs of
tuples that agree on all attributes common to the
two relations.
• In a theta-join , pairs of tuples are concatenated if
they meet a selection condition associated with the
theta-join.
Constraints in Relational Algebra
• Many common kinds of constraints can be expressed
as
– the containment of one relational algebra expression in
another , or
– as the equality of a relational algebra expression to the
empty set.