<s>
A	O
database	B-General_Concept
model	I-General_Concept
is	O
a	O
type	O
of	O
data	B-Application
model	I-Application
that	O
determines	O
the	O
logical	O
structure	O
of	O
a	O
database	O
.	O
</s>
<s>
The	O
most	O
popular	O
example	O
of	O
a	O
database	B-General_Concept
model	I-General_Concept
is	O
the	O
relational	B-Architecture
model	I-Architecture
,	O
which	O
uses	O
a	O
table-based	O
format	O
.	O
</s>
<s>
Common	O
logical	B-Application
data	I-Application
models	I-Application
for	O
databases	B-Application
include	O
:	O
</s>
<s>
It	O
is	O
the	O
oldest	O
form	O
of	O
data	B-Application
base	I-Application
model	O
.	O
</s>
<s>
It	O
was	O
developed	O
by	O
IBM	O
for	O
IMS	O
(	O
information	B-Application
Management	I-Application
System	I-Application
)	O
.	O
</s>
<s>
An	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
database	I-General_Concept
combines	O
the	O
two	O
related	O
structures	O
.	O
</s>
<s>
Physical	B-Application
data	I-Application
models	I-Application
include	O
:	O
</s>
<s>
A	O
given	O
database	B-Application
management	I-Application
system	I-Application
may	O
provide	O
one	O
or	O
more	O
models	O
.	O
</s>
<s>
Most	O
database	B-Application
management	I-Application
systems	I-Application
are	O
built	O
around	O
one	O
particular	O
data	B-Application
model	I-Application
,	O
although	O
it	O
is	O
possible	O
for	O
products	O
to	O
offer	O
support	O
for	O
more	O
than	O
one	O
model	O
.	O
</s>
<s>
Various	O
physical	B-Application
data	I-Application
models	I-Application
can	O
implement	O
any	O
given	O
logical	O
model	O
.	O
</s>
<s>
The	O
relational	B-Architecture
model	I-Architecture
,	O
for	O
example	O
,	O
defines	O
operations	O
such	O
as	O
select	B-Language
(	O
project	B-Algorithm
)	O
and	O
join	B-Language
.	O
</s>
<s>
Although	O
these	O
operations	O
may	O
not	O
be	O
explicit	O
in	O
a	O
particular	O
query	B-Language
language	I-Language
,	O
they	O
provide	O
the	O
foundation	O
on	O
which	O
a	O
query	B-Language
language	I-Language
is	O
built	O
.	O
</s>
<s>
This	O
tabular	O
format	O
is	O
a	O
precursor	O
to	O
the	O
relational	B-Architecture
model	I-Architecture
.	O
</s>
<s>
These	O
models	O
were	O
popular	O
in	O
the	O
1960s	O
,	O
1970s	O
,	O
but	O
nowadays	O
can	O
be	O
found	O
primarily	O
in	O
old	O
legacy	B-Device
systems	I-Device
.	O
</s>
<s>
They	O
are	O
characterized	O
primarily	O
by	O
being	O
navigational	B-General_Concept
with	O
strong	O
connections	O
between	O
their	O
logical	O
and	O
physical	O
representations	O
,	O
and	O
deficiencies	O
in	O
data	B-General_Concept
independence	I-General_Concept
.	O
</s>
<s>
In	O
a	O
hierarchical	B-General_Concept
model	I-General_Concept
,	O
data	O
is	O
organized	O
into	O
a	O
tree-like	B-Application
structure	I-Application
,	O
implying	O
a	O
single	O
parent	O
for	O
each	O
record	O
.	O
</s>
<s>
Hierarchical	O
structures	O
were	O
widely	O
used	O
in	O
the	O
early	O
mainframe	O
database	B-Application
management	I-Application
systems	I-Application
,	O
such	O
as	O
the	O
Information	B-Application
Management	I-Application
System	I-Application
(	O
IMS	O
)	O
by	O
IBM	O
,	O
and	O
now	O
describe	O
the	O
structure	O
of	O
XML	B-Protocol
documents	I-Protocol
.	O
</s>
<s>
This	O
structure	O
allows	O
one-to-many	B-Application
relationship	O
between	O
two	O
types	O
of	O
data	O
.	O
</s>
<s>
The	O
network	B-General_Concept
model	I-General_Concept
expands	O
upon	O
the	O
hierarchical	O
structure	O
,	O
allowing	O
many-to-many	O
relationships	O
in	O
a	O
tree-like	B-Application
structure	I-Application
that	O
allows	O
multiple	O
parents	O
.	O
</s>
<s>
It	O
was	O
most	O
popular	O
before	O
being	O
replaced	O
by	O
the	O
relational	B-Architecture
model	I-Architecture
,	O
and	O
is	O
defined	O
by	O
the	O
CODASYL	B-Language
specification	O
.	O
</s>
<s>
The	O
network	B-General_Concept
model	I-General_Concept
organizes	O
data	O
using	O
two	O
fundamental	O
concepts	O
,	O
called	O
records	O
and	O
sets	O
.	O
</s>
<s>
Records	O
contain	O
fields	O
(	O
which	O
may	O
be	O
organized	O
hierarchically	O
,	O
as	O
in	O
the	O
programming	O
language	O
COBOL	B-Application
)	O
.	O
</s>
<s>
Sets	O
(	O
not	O
to	O
be	O
confused	O
with	O
mathematical	O
sets	O
)	O
define	O
one-to-many	B-Application
relationships	O
between	O
records	O
:	O
one	O
owner	O
,	O
many	O
members	O
.	O
</s>
<s>
A	O
set	O
consists	O
of	O
circular	O
linked	B-Data_Structure
lists	I-Data_Structure
where	O
one	O
record	O
type	O
,	O
the	O
set	O
owner	O
or	O
parent	O
,	O
appears	O
once	O
in	O
each	O
circle	O
,	O
and	O
a	O
second	O
record	O
type	O
,	O
the	O
subordinate	O
or	O
child	O
,	O
may	O
appear	O
multiple	O
times	O
in	O
each	O
circle	O
.	O
</s>
<s>
Access	O
to	O
records	O
is	O
either	O
sequential	O
(	O
usually	O
in	O
each	O
record	O
type	O
)	O
or	O
by	O
navigation	O
in	O
the	O
circular	O
linked	B-Data_Structure
lists	I-Data_Structure
.	O
</s>
<s>
The	O
network	B-General_Concept
model	I-General_Concept
is	O
able	O
to	O
represent	O
redundancy	O
in	O
data	O
more	O
efficiently	O
than	O
in	O
the	O
hierarchical	B-General_Concept
model	I-General_Concept
,	O
and	O
there	O
can	O
be	O
more	O
than	O
one	O
path	O
from	O
an	O
ancestor	B-Application
node	I-Application
to	O
a	O
descendant	O
.	O
</s>
<s>
The	O
operations	O
of	O
the	O
network	B-General_Concept
model	I-General_Concept
are	O
navigational	B-General_Concept
in	O
style	O
:	O
a	O
program	O
maintains	O
a	O
current	O
position	O
,	O
and	O
navigates	O
from	O
one	O
record	O
to	O
another	O
by	O
following	O
the	O
relationships	O
in	O
which	O
the	O
record	O
participates	O
.	O
</s>
<s>
Although	O
it	O
is	O
not	O
an	O
essential	O
feature	O
of	O
the	O
model	O
,	O
network	B-General_Concept
databases	I-General_Concept
generally	O
implement	O
the	O
set	O
relationships	O
by	O
means	O
of	O
pointers	O
that	O
directly	O
address	O
the	O
location	O
of	O
a	O
record	O
on	O
disk	O
.	O
</s>
<s>
Popular	O
DBMS	O
products	O
that	O
utilized	O
it	O
were	O
Cincom	B-Application
Systems	I-Application
 '	O
Total	O
and	O
Cullinet	O
's	O
IDMS	B-Application
.	O
</s>
<s>
IDMS	B-Application
gained	O
a	O
considerable	O
customer	O
base	O
;	O
in	O
the	O
1980s	O
,	O
it	O
adopted	O
the	O
relational	B-Architecture
model	I-Architecture
and	O
SQL	B-Language
in	O
addition	O
to	O
its	O
original	O
tools	O
and	O
languages	O
.	O
</s>
<s>
Most	O
object	B-Application
databases	I-Application
(	O
invented	O
in	O
the	O
1990s	O
)	O
use	O
the	O
navigational	B-General_Concept
concept	O
to	O
provide	O
fast	O
navigation	O
across	O
networks	O
of	O
objects	O
,	O
generally	O
using	O
object	O
identifiers	O
as	O
"	O
smart	O
"	O
pointers	O
to	O
related	O
objects	O
.	O
</s>
<s>
Objectivity/DB	B-Application
,	O
for	O
instance	O
,	O
implements	O
named	O
one-to-one	O
,	O
one-to-many	B-Application
,	O
many-to-one	B-Application
,	O
and	O
many-to-many	O
named	O
relationships	O
that	O
can	O
cross	O
databases	B-Application
.	O
</s>
<s>
Many	O
object	B-Application
databases	I-Application
also	O
support	O
SQL	B-Language
,	O
combining	O
the	O
strengths	O
of	O
both	O
models	O
.	O
</s>
<s>
In	O
an	O
inverted	B-Algorithm
file	I-Algorithm
or	O
inverted	B-Algorithm
index	I-Algorithm
,	O
the	O
contents	O
of	O
the	O
data	O
are	O
used	O
as	O
keys	O
in	O
a	O
lookup	O
table	O
,	O
and	O
the	O
values	O
in	O
the	O
table	O
are	O
pointers	O
to	O
the	O
location	O
of	O
each	O
instance	O
of	O
a	O
given	O
content	O
item	O
.	O
</s>
<s>
This	O
is	O
also	O
the	O
logical	O
structure	O
of	O
contemporary	O
database	B-Data_Structure
indexes	I-Data_Structure
,	O
which	O
might	O
only	O
use	O
the	O
contents	O
from	O
a	O
particular	O
columns	O
in	O
the	O
lookup	O
table	O
.	O
</s>
<s>
The	O
inverted	B-Algorithm
file	I-Algorithm
data	B-Application
model	I-Application
can	O
put	O
indexes	O
in	O
a	O
set	O
of	O
files	O
next	O
to	O
existing	O
flat	B-Application
database	I-Application
files	O
,	O
in	O
order	O
to	O
efficiently	O
directly	O
access	O
needed	O
records	O
in	O
these	O
files	O
.	O
</s>
<s>
Notable	O
for	O
using	O
this	O
data	B-Application
model	I-Application
is	O
the	O
ADABAS	B-Application
DBMS	O
of	O
Software	O
AG	O
,	O
introduced	O
in	O
1970	O
.	O
</s>
<s>
ADABAS	B-Application
has	O
gained	O
considerable	O
customer	O
base	O
and	O
exists	O
and	O
supported	O
until	O
today	O
.	O
</s>
<s>
In	O
the	O
1980s	O
it	O
has	O
adopted	O
the	O
relational	B-Architecture
model	I-Architecture
and	O
SQL	B-Language
in	O
addition	O
to	O
its	O
original	O
tools	O
and	O
languages	O
.	O
</s>
<s>
Document-oriented	B-General_Concept
database	I-General_Concept
Clusterpoint	B-Operating_System
uses	O
inverted	O
indexing	O
model	O
to	O
provide	O
fast	O
full-text	B-Application
search	I-Application
for	O
XML	B-Protocol
or	O
JSON	B-General_Concept
data	O
objects	O
for	O
example	O
.	O
</s>
<s>
The	O
relational	B-Architecture
model	I-Architecture
was	O
introduced	O
by	O
E.F.	O
Codd	O
in	O
1970	O
as	O
a	O
way	O
to	O
make	O
database	B-Application
management	I-Application
systems	I-Application
more	O
independent	O
of	O
any	O
particular	O
application	O
.	O
</s>
<s>
The	O
products	O
that	O
are	O
generally	O
referred	O
to	O
as	O
relational	B-Application
databases	I-Application
in	O
fact	O
implement	O
a	O
model	O
that	O
is	O
only	O
an	O
approximation	O
to	O
the	O
mathematical	O
model	O
defined	O
by	O
Codd	O
.	O
</s>
<s>
Three	O
key	O
terms	O
are	O
used	O
extensively	O
in	O
relational	B-Architecture
database	I-Architecture
models	I-Architecture
:	O
relations	B-Language
,	O
attributes	O
,	O
and	O
domains	O
.	O
</s>
<s>
A	O
relation	B-Language
is	O
a	O
table	O
with	O
columns	O
and	O
rows	O
.	O
</s>
<s>
The	O
named	O
columns	O
of	O
the	O
relation	B-Language
are	O
called	O
attributes	O
,	O
and	O
the	O
domain	B-Application
is	O
the	O
set	O
of	O
values	O
the	O
attributes	O
are	O
allowed	O
to	O
take	O
.	O
</s>
<s>
The	O
basic	O
data	O
structure	O
of	O
the	O
relational	B-Architecture
model	I-Architecture
is	O
the	O
table	O
,	O
where	O
information	O
about	O
a	O
particular	O
entity	O
(	O
say	O
,	O
an	O
employee	O
)	O
is	O
represented	O
in	O
rows	O
(	O
also	O
called	O
tuples	B-Application
)	O
and	O
columns	O
.	O
</s>
<s>
Thus	O
,	O
the	O
"	O
relation	B-Language
"	O
in	O
"	O
relational	B-Application
database	I-Application
"	O
refers	O
to	O
the	O
various	O
tables	O
in	O
the	O
database	O
;	O
a	O
relation	B-Language
is	O
a	O
set	O
of	O
tuples	B-Application
.	O
</s>
<s>
The	O
columns	O
enumerate	O
the	O
various	O
attributes	O
of	O
the	O
entity	O
(	O
the	O
employee	O
's	O
name	O
,	O
address	O
or	O
phone	O
number	O
,	O
for	O
example	O
)	O
,	O
and	O
a	O
row	O
is	O
an	O
actual	O
instance	O
of	O
the	O
entity	O
(	O
a	O
specific	O
employee	O
)	O
that	O
is	O
represented	O
by	O
the	O
relation	B-Language
.	O
</s>
<s>
As	O
a	O
result	O
,	O
each	O
tuple	B-Application
of	O
the	O
employee	O
table	O
represents	O
various	O
attributes	O
of	O
a	O
single	O
employee	O
.	O
</s>
<s>
All	O
relations	B-Language
(	O
and	O
,	O
thus	O
,	O
tables	O
)	O
in	O
a	O
relational	B-Application
database	I-Application
have	O
to	O
adhere	O
to	O
some	O
basic	O
rules	O
to	O
qualify	O
as	O
relations	B-Language
.	O
</s>
<s>
Second	O
,	O
there	O
ca	O
n't	O
be	O
identical	O
tuples	B-Application
or	O
rows	O
in	O
a	O
table	O
.	O
</s>
<s>
And	O
third	O
,	O
each	O
tuple	B-Application
will	O
contain	O
a	O
single	O
value	O
for	O
each	O
of	O
its	O
attributes	O
.	O
</s>
<s>
A	O
relational	B-Application
database	I-Application
contains	O
multiple	O
tables	O
,	O
each	O
similar	O
to	O
the	O
one	O
in	O
the	O
"	O
flat	O
"	O
database	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
One	O
of	O
the	O
strengths	O
of	O
the	O
relational	B-Architecture
model	I-Architecture
is	O
that	O
,	O
in	O
principle	O
,	O
any	O
value	O
occurring	O
in	O
two	O
different	O
records	O
(	O
belonging	O
to	O
the	O
same	O
table	O
or	O
to	O
different	O
tables	O
)	O
,	O
implies	O
a	O
relationship	O
among	O
those	O
two	O
records	O
.	O
</s>
<s>
Yet	O
,	O
in	O
order	O
to	O
enforce	O
explicit	O
integrity	O
constraints	O
,	O
relationships	O
between	O
records	O
in	O
tables	O
can	O
also	O
be	O
defined	O
explicitly	O
,	O
by	O
identifying	O
or	O
non-identifying	O
parent-child	O
relationships	O
characterized	O
by	O
assigning	O
cardinality	O
(	O
1:1	O
,	O
(	O
0	O
)	O
1:M	O
,	O
M:M	O
)	O
.	O
</s>
<s>
Tables	O
can	O
also	O
have	O
a	O
designated	O
single	O
attribute	O
or	O
a	O
set	O
of	O
attributes	O
that	O
can	O
act	O
as	O
a	O
"	O
key	O
"	O
,	O
which	O
can	O
be	O
used	O
to	O
uniquely	O
identify	O
each	O
tuple	B-Application
in	O
the	O
table	O
.	O
</s>
<s>
Keys	O
are	O
commonly	O
used	O
to	O
join	B-Language
or	O
combine	O
data	O
from	O
two	O
or	O
more	O
tables	O
.	O
</s>
<s>
A	O
key	O
that	O
has	O
an	O
external	O
,	O
real-world	O
meaning	O
(	O
such	O
as	O
a	O
person	O
's	O
name	O
,	O
a	O
book	O
's	O
ISBN	B-Library
,	O
or	O
a	O
car	O
's	O
serial	O
number	O
)	O
is	O
sometimes	O
called	O
a	O
"	O
natural	O
"	O
key	O
.	O
</s>
<s>
In	O
practice	O
,	O
most	O
databases	B-Application
have	O
both	O
generated	O
and	O
natural	O
keys	O
,	O
because	O
generated	O
keys	O
can	O
be	O
used	O
internally	O
to	O
create	O
links	O
between	O
rows	O
that	O
cannot	O
break	O
,	O
while	O
natural	O
keys	O
can	O
be	O
used	O
,	O
less	O
reliably	O
,	O
for	O
searches	O
and	O
for	O
integration	O
with	O
other	O
databases	B-Application
.	O
</s>
<s>
(	O
For	O
example	O
,	O
records	O
in	O
two	O
independently	O
developed	O
databases	B-Application
could	O
be	O
matched	O
up	O
by	O
social	O
security	O
number	O
,	O
except	O
when	O
the	O
social	O
security	O
numbers	O
are	O
incorrect	O
,	O
missing	O
,	O
or	O
have	O
changed	O
.	O
)	O
</s>
<s>
The	O
most	O
common	O
query	B-Language
language	I-Language
used	O
with	O
the	O
relational	B-Architecture
model	I-Architecture
is	O
the	O
Structured	B-Language
Query	I-Language
Language	I-Language
(	O
SQL	B-Language
)	O
.	O
</s>
<s>
The	O
dimensional	B-General_Concept
model	I-General_Concept
is	O
a	O
specialized	O
adaptation	O
of	O
the	O
relational	B-Architecture
model	I-Architecture
used	O
to	O
represent	O
data	O
in	O
data	O
warehouses	O
in	O
a	O
way	O
that	O
data	O
can	O
be	O
easily	O
summarized	O
using	O
online	B-Application
analytical	I-Application
processing	I-Application
,	O
or	O
OLAP	B-Application
queries	B-Language
.	O
</s>
<s>
In	O
the	O
dimensional	B-General_Concept
model	I-General_Concept
,	O
a	O
database	O
schema	O
consists	O
of	O
a	O
single	O
large	O
table	O
of	O
facts	O
that	O
are	O
described	O
using	O
dimensions	O
and	O
measures	O
.	O
</s>
<s>
A	O
dimension	O
provides	O
the	O
context	O
of	O
a	O
fact	O
(	O
such	O
as	O
who	O
participated	O
,	O
when	O
and	O
where	O
it	O
happened	O
,	O
and	O
its	O
type	O
)	O
and	O
is	O
used	O
in	O
queries	B-Language
to	O
group	O
related	O
facts	O
together	O
.	O
</s>
<s>
In	O
an	O
OLAP	B-Application
query	B-Language
,	O
dimensions	O
are	O
chosen	O
and	O
the	O
facts	O
are	O
grouped	O
and	O
aggregated	O
together	O
to	O
create	O
a	O
summary	O
.	O
</s>
<s>
The	O
dimensional	B-General_Concept
model	I-General_Concept
is	O
often	O
implemented	O
on	O
top	O
of	O
the	O
relational	B-Architecture
model	I-Architecture
using	O
a	O
star	B-Application
schema	I-Application
,	O
consisting	O
of	O
one	O
highly	O
normalized	O
table	O
containing	O
the	O
facts	O
,	O
and	O
surrounding	O
denormalized	O
tables	O
containing	O
each	O
dimension	O
.	O
</s>
<s>
An	O
alternative	O
physical	O
implementation	O
,	O
called	O
a	O
snowflake	B-Language
schema	I-Language
,	O
normalizes	O
multi-level	O
hierarchies	O
within	O
a	O
dimension	O
into	O
multiple	O
tables	O
.	O
</s>
<s>
Coming	O
up	O
with	O
a	O
standard	O
set	O
of	O
dimensions	O
is	O
an	O
important	O
part	O
of	O
dimensional	B-Application
modeling	I-Application
.	O
</s>
<s>
Its	O
high	O
performance	O
has	O
made	O
the	O
dimensional	B-General_Concept
model	I-General_Concept
the	O
most	O
popular	O
database	O
structure	O
for	O
OLAP	B-Application
.	O
</s>
<s>
Products	O
offering	O
a	O
more	O
general	O
data	B-Application
model	I-Application
than	O
the	O
relational	B-Architecture
model	I-Architecture
are	O
sometimes	O
classified	O
as	O
post-relational	O
.	O
</s>
<s>
Some	O
of	O
these	O
extensions	O
to	O
the	O
relational	B-Architecture
model	I-Architecture
integrate	O
concepts	O
from	O
technologies	O
that	O
pre-date	O
the	O
relational	B-Architecture
model	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
they	O
allow	O
representation	O
of	O
a	O
directed	O
graph	O
with	O
trees	B-Application
on	O
the	O
nodes	O
.	O
</s>
<s>
The	O
German	O
company	O
sones	O
implements	O
this	O
concept	O
in	O
its	O
GraphDB	B-Application
.	O
</s>
<s>
Paradoxically	O
,	O
this	O
allows	O
products	O
that	O
are	O
historically	O
pre-relational	O
,	O
such	O
as	O
PICK	B-Application
and	O
MUMPS	B-Language
,	O
to	O
make	O
a	O
plausible	O
claim	O
to	O
be	O
post-relational	O
.	O
</s>
<s>
Graph	B-Application
databases	I-Application
allow	O
even	O
more	O
general	O
structure	O
than	O
a	O
network	B-General_Concept
database	I-General_Concept
;	O
any	O
node	O
may	O
be	O
connected	O
to	O
any	O
other	O
node	O
.	O
</s>
<s>
Multivalue	B-Application
databases	I-Application
are	O
"	O
lumpy	O
"	O
data	O
,	O
in	O
that	O
they	O
can	O
store	O
exactly	O
the	O
same	O
way	O
as	O
relational	B-Application
databases	I-Application
,	O
but	O
they	O
also	O
permit	O
a	O
level	O
of	O
depth	O
which	O
the	O
relational	B-Architecture
model	I-Architecture
can	O
only	O
approximate	O
using	O
sub-tables	O
.	O
</s>
<s>
This	O
is	O
nearly	O
identical	O
to	O
the	O
way	O
XML	B-Protocol
expresses	O
data	O
,	O
where	O
a	O
given	O
field/attribute	O
can	O
have	O
multiple	O
right	O
answers	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Multivalue	B-Application
can	O
be	O
thought	O
of	O
as	O
a	O
compressed	O
form	O
of	O
XML	B-Protocol
.	O
</s>
<s>
An	O
example	O
is	O
an	O
invoice	O
,	O
which	O
in	O
either	O
multivalue	B-Application
or	O
relational	O
data	O
could	O
be	O
seen	O
as	O
(	O
A	O
)	O
Invoice	O
Header	O
Table	O
-	O
one	O
entry	O
per	O
invoice	O
,	O
and	O
(	O
B	O
)	O
Invoice	O
Detail	O
Table	O
-	O
one	O
entry	O
per	O
line	O
item	O
.	O
</s>
<s>
In	O
the	O
multivalue	B-Application
model	I-Application
,	O
we	O
have	O
the	O
option	O
of	O
storing	O
the	O
data	O
as	O
on	O
table	O
,	O
with	O
an	O
embedded	O
table	O
to	O
represent	O
the	O
detail	O
:	O
(	O
A	O
)	O
Invoice	O
Table	O
-	O
one	O
entry	O
per	O
invoice	O
,	O
no	O
other	O
tables	O
needed	O
.	O
</s>
<s>
In	O
the	O
1990s	O
,	O
the	O
object-oriented	B-Language
programming	I-Language
paradigm	O
was	O
applied	O
to	O
database	O
technology	O
,	O
creating	O
a	O
new	O
database	B-General_Concept
model	I-General_Concept
known	O
as	O
object	B-Application
databases	I-Application
.	O
</s>
<s>
This	O
aims	O
to	O
avoid	O
the	O
object	B-Architecture
–	I-Architecture
relational	I-Architecture
impedance	I-Architecture
mismatch	I-Architecture
–	O
the	O
overhead	O
of	O
converting	O
information	O
between	O
its	O
representation	O
in	O
the	O
database	O
(	O
for	O
example	O
as	O
rows	O
in	O
tables	O
)	O
and	O
its	O
representation	O
in	O
the	O
application	O
program	O
(	O
typically	O
as	O
objects	O
)	O
.	O
</s>
<s>
Object	B-Application
databases	I-Application
also	O
introduce	O
the	O
key	O
ideas	O
of	O
object	O
programming	O
,	O
such	O
as	O
encapsulation	B-Application
and	O
polymorphism	B-Application
,	O
into	O
the	O
world	O
of	O
databases	B-Application
.	O
</s>
<s>
Some	O
products	O
have	O
approached	O
the	O
problem	O
from	O
the	O
application	O
programming	O
end	O
,	O
by	O
making	O
the	O
objects	O
manipulated	O
by	O
the	O
program	O
persistent	B-Application
.	O
</s>
<s>
This	O
typically	O
requires	O
the	O
addition	O
of	O
some	O
kind	O
of	O
query	B-Language
language	I-Language
,	O
since	O
conventional	O
programming	O
languages	O
do	O
not	O
have	O
the	O
ability	O
to	O
find	O
objects	O
based	O
on	O
their	O
information	O
content	O
.	O
</s>
<s>
Others	O
have	O
attacked	O
the	O
problem	O
from	O
the	O
database	O
end	O
,	O
by	O
defining	O
an	O
object-oriented	B-Language
data	B-Application
model	I-Application
for	O
the	O
database	O
,	O
and	O
defining	O
a	O
database	B-Application
programming	I-Application
language	O
that	O
allows	O
full	O
programming	O
capabilities	O
as	O
well	O
as	O
traditional	O
query	B-Language
facilities	O
.	O
</s>
<s>
Object	B-Application
databases	I-Application
suffered	O
because	O
of	O
a	O
lack	O
of	O
standardization	O
:	O
although	O
standards	O
were	O
defined	O
by	O
ODMG	B-Application
,	O
they	O
were	O
never	O
implemented	O
well	O
enough	O
to	O
ensure	O
interoperability	O
between	O
products	O
.	O
</s>
<s>
Nevertheless	O
,	O
object	B-Application
databases	I-Application
have	O
been	O
used	O
successfully	O
in	O
many	O
applications	O
:	O
usually	O
specialized	O
applications	O
such	O
as	O
engineering	O
databases	B-Application
or	O
molecular	O
biology	O
databases	B-Application
rather	O
than	O
mainstream	O
commercial	O
data	O
processing	O
.	O
</s>
<s>
However	O
,	O
object	B-Application
database	I-Application
ideas	O
were	O
picked	O
up	O
by	O
the	O
relational	O
vendors	O
and	O
influenced	O
extensions	O
made	O
to	O
these	O
products	O
and	O
indeed	O
to	O
the	O
SQL	B-Language
language	I-Language
.	O
</s>
<s>
An	O
alternative	O
to	O
translating	O
between	O
objects	O
and	O
relational	B-Application
databases	I-Application
is	O
to	O
use	O
an	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
mapping	I-General_Concept
(	O
ORM	O
)	O
library	O
.	O
</s>
