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4
RELATIONAL ALGEBRA AND
CALCULUS
Exercise 4.1
Explain the statement that relational algebra operators can be
com
posed
. Why is the ability to compose operators important?
Answer 4.1
Every operator in relational algebra accepts one or more relation in
stances as arguments and the result is always an relation instance. So the argument
of one operator could be the result of another operator. This is important because,
this makes it easy to write complex queries by simply composing the relational algebra
operators.
Exercise 4.2
Given two relations
R
1and
R
2, where
R
1 contains N1 tuples,
R
2con
tains N2 tuples, and N2
>
N1
>
0, give the minimum and maximum possible sizes (in
tuples) for the resulting relation produced by each of the following relational algebra
expressions. In each case, state any assumptions about the schemas for
R
1and
R
2
needed to make the expression meaningful:
(1)
R
1
∪
R
2, (2)
R
1
∩
R
2, (3)
R
1
−
R
2, (4)
R
1
×
R
2, (5)
σ
a
=5
(
R
1), (6)
π
a
(
R
1),
and (7)
R
1
/R
2
Answer 4.2
Answer omitted.
Exercise 4.3
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This note was uploaded on 01/17/2012 for the course EGN 4302 taught by Professor Dr.vishak during the Fall '12 term at University of Central Florida.
 Fall '12
 Dr.Vishak

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