<s>
Data	B-General_Concept
independence	I-General_Concept
is	O
the	O
type	O
of	O
data	O
transparency	O
that	O
matters	O
for	O
a	O
centralized	O
DBMS	B-Application
.	O
</s>
<s>
It	O
refers	O
to	O
the	O
immunity	O
of	O
user	O
applications	B-Application
to	O
changes	O
made	O
in	O
the	O
definition	O
and	O
organization	O
of	O
data	O
.	O
</s>
<s>
Application	B-Application
programs	I-Application
should	O
not	O
,	O
ideally	O
,	O
be	O
exposed	O
to	O
details	O
of	O
data	O
representation	O
and	O
storage	O
.	O
</s>
<s>
The	O
DBMS	B-Application
provides	O
an	O
abstract	O
view	O
of	O
the	O
data	O
that	O
hides	O
such	O
details	O
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
data	B-General_Concept
independence	I-General_Concept
:	O
physical	O
and	O
logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
The	O
data	B-General_Concept
independence	I-General_Concept
and	O
operation	O
independence	O
together	O
gives	O
the	O
feature	O
of	O
data	B-Application
abstraction	I-Application
.	O
</s>
<s>
There	O
are	O
two	O
levels	O
of	O
data	B-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
In	O
general	O
,	O
if	O
a	O
user	O
application	O
operates	O
on	O
a	O
subset	O
of	O
the	O
attributes	B-General_Concept
of	O
a	O
relation	B-Language
,	O
it	O
should	O
not	O
be	O
affected	O
later	O
when	O
new	O
attributes	B-General_Concept
are	O
added	O
to	O
the	O
same	O
relation	B-Language
.	O
</s>
<s>
Logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
indicates	O
that	O
the	O
conceptual	O
schema	O
can	O
be	O
changed	O
without	O
affecting	O
the	O
existing	O
schemas	O
.	O
</s>
<s>
Physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
deals	O
with	O
hiding	O
the	O
details	O
of	O
the	O
storage	O
structure	O
from	O
user	O
applications	B-Application
.	O
</s>
<s>
There	O
are	O
three	O
types	O
of	O
data	B-General_Concept
independence	I-General_Concept
:	O
</s>
<s>
Logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
:	O
The	O
ability	O
to	O
change	O
the	O
logical	O
(	O
conceptual	O
)	O
schema	O
without	O
changing	O
the	O
External	O
schema	O
(	O
User	O
View	O
)	O
is	O
called	O
logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
addition	O
or	O
removal	O
of	O
new	O
entities	O
,	O
attributes	B-General_Concept
,	O
or	O
relationships	O
to	O
the	O
conceptual	O
schema	O
or	O
having	O
to	O
rewrite	O
existing	O
application	B-Application
programs	I-Application
.	O
</s>
<s>
Physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
:	O
The	O
ability	O
to	O
change	O
the	O
physical	O
schema	O
without	O
changing	O
the	O
logical	O
schema	O
is	O
called	O
physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
View	O
level	O
data	B-General_Concept
independence	I-General_Concept
:	O
always	O
independent	O
no	O
effect	O
,	O
because	O
there	O
does	O
n't	O
exist	O
any	O
other	O
level	O
above	O
view	O
level	O
.	O
</s>
<s>
Data	B-General_Concept
independence	I-General_Concept
can	O
be	O
explained	O
as	O
follows	O
:	O
Each	O
higher	O
level	O
of	O
the	O
data	O
architecture	O
is	O
immune	O
to	O
changes	O
of	O
the	O
next	O
lower	O
level	O
of	O
the	O
architecture	O
.	O
</s>
<s>
Physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
is	O
present	O
in	O
most	O
databases	B-Application
and	O
file	O
environment	O
in	O
which	O
hardware	O
storage	O
of	O
encoding	O
,	O
exact	O
location	O
of	O
data	O
on	O
disk	O
,	O
merging	O
of	O
records	O
,	O
so	O
on	O
this	O
are	O
hidden	O
from	O
user	O
.	O
</s>
<s>
The	O
ability	O
to	O
modify	O
schema	O
definition	O
in	O
one	O
level	O
without	O
affecting	O
schema	O
of	O
that	O
definition	O
in	O
the	O
next	O
higher	O
level	O
is	O
called	O
data	B-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
There	O
are	O
two	O
levels	O
of	O
data	B-General_Concept
independence	I-General_Concept
,	O
they	O
are	O
Physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
and	O
Logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
Physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
is	O
the	O
ability	O
to	O
modify	O
the	O
physical	O
schema	O
without	O
causing	O
application	B-Application
programs	I-Application
to	O
be	O
rewritten	O
.	O
</s>
<s>
Logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
is	O
the	O
ability	O
to	O
modify	O
the	O
logical	O
schema	O
without	O
causing	O
application	B-Application
programs	I-Application
to	O
be	O
rewritten	O
.	O
</s>
<s>
Logical	B-General_Concept
Data	I-General_Concept
independence	I-General_Concept
means	O
if	O
we	O
add	O
some	O
new	O
columns	O
or	O
remove	O
some	O
columns	O
from	O
table	O
then	O
the	O
user	O
view	O
and	O
programs	O
should	O
not	O
change	O
.	O
</s>
<s>
Logical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
is	O
more	O
difficult	O
to	O
achieve	O
than	O
physical	B-General_Concept
data	I-General_Concept
independence	I-General_Concept
,	O
since	O
application	B-Application
programs	I-Application
are	O
heavily	O
dependent	O
on	O
the	O
logical	O
structure	O
of	O
the	O
data	O
that	O
they	O
access	O
.	O
</s>
