chapter07 实践习题2
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C H A P T E R
7
Relational Database Design
Solutions for the Pratie Exerises of Chapter 7
Pratie Exerises
7.1
Answer :
A deomposition
^
R
1
,
R
2
`
is a lossless deomposition if
R
1
ã
R
2
R
1
or
R
1
ã
R
2
R
2
. Let
R
1
=
(
A
,
B
,
C
),
R
2
=
(
A
,
D
,
E
), and
R
1
ã
R
2
=
A
.
Sine
A
is a andidate key (see Pratie Exerise 7.6),
R
1
ã
R
2
R
1
.
7.2
Answer :
The nontrivial funtional dependenies are:
A
B
and
C
B
, and a
dependeny they logially imply:
AC
B
.
C
does not funtionally determine
A
beause the r st and third tuples have t he same
C
but dierent
A
values. The
same tuples also show
B
does not funtionally deter mine
A
. Likewise,
A
does not
funtionally deter mine
C
beause the rst two tuples have the same
A
value and
dierent
C
values. The same tuples also show
B
does not funtionally deter mine
C
. There are 19 trivial funtional dependenies of the form
, where
Ó
.
7.3
Answer :
Let
Pk
(
r
) denote the pr imar y key attribute of relation
r
.
The funtional dependenies
Pk
(
student
)
Pk
(
instrutor
) and
Pk
(
instrutor
)
Pk
(
student
) indiate a one-to-one relationship be-
ause any two tuples with the same value for
student
must have the same
value for
instrutor
, and any two tuples ag reeing on
instrutor
must have
the same value for
student
.
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