<s>
In	O
database	O
modeling	O
,	O
a	O
first	B-Application
class	I-Application
item	O
is	O
one	O
that	O
has	O
an	O
identity	B-Application
independent	O
of	O
any	O
other	O
item	O
.	O
</s>
<s>
The	O
identity	B-Application
allows	O
the	O
item	O
to	O
persist	O
when	O
its	O
attributes	O
change	O
,	O
and	O
allows	O
other	O
items	O
to	O
claim	O
relationships	O
with	O
the	O
item	O
.	O
</s>
<s>
As	O
a	O
general	O
rule	O
,	O
first	B-Application
class	I-Application
items	O
represent	O
things	O
rather	O
than	O
relationships	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
database	O
representations	O
of	O
a	O
human	O
and	O
of	O
a	O
company	O
are	O
each	O
first	B-Application
class	I-Application
items	O
.	O
</s>
<s>
However	O
,	O
the	O
fact	O
that	O
the	O
person	O
is	O
an	O
employee	O
of	O
that	O
company	O
is	O
not	O
a	O
first	B-Application
class	I-Application
item	O
.	O
</s>
<s>
information	O
about	O
the	O
salary	O
the	O
company	O
pays	O
to	O
its	O
employee	O
,	O
is	O
not	O
a	O
first	B-Application
class	I-Application
item	O
.	O
</s>
<s>
Typically	O
,	O
a	O
relational	B-Application
database	I-Application
will	O
include	O
several	O
tables	O
,	O
each	O
of	O
which	O
contains	O
rows	B-Application
representing	O
first	B-Application
class	I-Application
items	O
of	O
a	O
given	O
type	O
(	O
e.g.	O
</s>
<s>
a	O
table	B-Application
of	O
people	O
,	O
a	O
table	B-Application
of	O
companies	O
)	O
.	O
</s>
<s>
It	O
will	O
also	O
contain	O
other	O
tables	O
representing	O
relationships	O
between	O
these	O
first	B-Application
class	I-Application
items	O
.	O
</s>
<s>
In	O
a	O
table	B-Application
representing	O
first	B-Application
class	I-Application
items	O
,	O
one	O
column	O
of	O
the	O
table	B-Application
will	O
typically	O
contain	O
a	O
different	O
integer	O
assigned	O
to	O
each	O
row	O
(	O
effectively	O
,	O
to	O
each	O
item	O
)	O
as	O
a	O
unique	O
identifier	O
:	O
that	O
is	O
to	O
say	O
,	O
unique	O
for	O
objects	O
of	O
this	O
type	O
;	O
objects	O
of	O
different	O
types	O
,	O
represented	O
in	O
different	O
tables	O
,	O
can	O
coincidentally	O
have	O
the	O
same	O
identifier	O
,	O
but	O
the	O
coincidence	O
is	O
meaningless	O
.	O
</s>
<s>
In	O
a	O
relational	B-Application
database	I-Application
,	O
a	O
table	B-Application
representing	O
a	O
relationship	O
between	O
two	O
or	O
more	O
first	B-Application
class	I-Application
items	O
(	O
or	O
data	O
about	O
that	O
relationship	O
)	O
will	O
usually	O
not	O
have	O
special	O
identifiers	O
for	O
its	O
rows	B-Application
.	O
</s>
<s>
Instead	O
these	O
rows	B-Application
will	O
be	O
identified	O
by	O
an	O
ordered	O
tuple	B-Application
consisting	O
of	O
unique	O
identifiers	O
of	O
the	O
first	B-Application
class	I-Application
items	O
involved	O
in	O
the	O
relationship	O
.	O
</s>
