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da questo dispositivo ha anche altre istruzioni :
Facilità d'uso
Rollforw ard
reco
v
ery
preserv
es
logical
and
ph
ysical
data
in
tegrit
y
due
to
a
hard
crash.
Dynamic
restructuring
allo ws
y
ou
to
c
hange
data
structure,
table
capacities,
and
securit
y
without
unloading
and
loading
the
database.
Null
data
v
alues
allo w
use
of
elds
that
are
relev
an
t
to
some,
but
not
all,
records
in
a
table.
HP
SQL
do es
not
require
y
ou
to
dene
explicit
relationships
b et
w
een
data
sets.
When
y
ou
p erform
a
query
,
it
determines
relationships
b
y
matching
v
alues
b et
w
een
elds
common
to
t
w
o
or
more
data
sets.
If
y
ou
consider
a
data
set
to
b e
analogous
to
a
table,
a
given
eld
in
the
data
set
w
ould
o ccur
as
a
column
in
the
table.
Data
from
an
y
n
um
b er
of
tables
that
share
a
common
column
can
b e
related,
as
needed,
and
y
ou
can
dene
them
to
b e
an
HP
SQL
database.
The
query
optimizer
determines
the
b est
data
access
strategy
based
on
factors
suc
h
as
the
presence
of
indexes
and
the
relativ e
sizes
of
accessed
tables.
In
teractive
SQL
(ISQL)
is
the
in
teractive
in
terface
to
HP
SQL.
It
pro
vides
the
functionalit
y
of
a
data
denition
language
(DDL)
and
a
data
manipulation
language
(SQL).
DDL
allo
ws
the
database
administrator
to
con
trol
all
asp ects
of
database
creation
and
mo
dication.
SQL
allo
ws
the
programmer
or
frequen
t
user
to
in
teractively
query
a
database.
Security
Securit
y
is
maintained
b
y
allo
wing
sp ecication
of
appropriate
lev
els
of
access
privileges
to
individual
users
or
groups
of
users.
HP
SQL
allo ws
read
(SELECT)
access
and
write
access
privileges
to
b e
assigned
at
the
table
lev
el.
Read
or
write
access
restriction
at
a
ner
granularit
y
than
the
table
lev
el,
suc
h
as
at
the
column
lev
el,
may
b e
obtained
b
y
dening
a
view
of
the
table
that
omits
the
sensitive
information.
Mo
dication
(UPDA
TE)
authorit
y
may
b e
granted
at
the
column
lev
el
without
requiring
that
a
view
b e
sp ecied.
W
rite
access
may
b e
assigned
to
allo w
an
y
com
bination
of
the
follo wing
capabilities:
Ro
w
mo
dication
(UPDA
TE)
Ro
w
insertion
(INSER
T)
Ro
w
deletion
(DELETE)
The
database
administrator
(DBA)
assigns
access
privileges
b
y
grouping
users
with
common
access
needs
in
to
authorization
groups.
The
particular
read
and
write
authorities
are
then
assigned
b
y
the
DBA
to
eac
h
of
these
groups.
Users
with
unique
access
requiremen
ts
may
also
b e
granted
privileges
directly
.
Logical
Transaction
A
logical
transaction
is
a
series
of
database
mo
dications
of
whic
h
either
all
or
none
m
ust
b e
p erformed
to
leav
e
the
database
in
a
consisten
t
state.
The
particular
grouping
of
mo
dications
dened
b
y
the
user
to
b e
a
transaction
v
aries
dep ending
on
the
particular
application.
An
example
of
a
transaction
is
an
accoun
ting
en
try
to
pa
y
a
bill:
the
credit
to
the
cash
accoun
t
and
debit
to
the
accoun
ts
pa
y
able
accoun
t
m
ust
b oth
b e
p erformed
to
a
v
oid
leaving
the
accoun
ts
out
of
balance.
This
concept
of
a
logical
transaction
is
essen
tial
to
ensuring
that
data
in
tegrit
y
is
maintained
when
m
ultiple
users
are
concurren
tly
accessing
the
database
or
in
the
ev
en
t
of
system
failure.
Data
Management
7-7
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