<s>
Relational	B-Application
Model/Tasmania	I-Application
(	O
RM/T	O
)	O
was	O
published	O
by	O
Edgar	O
F	O
.	O
Codd	O
in	O
1979	O
and	O
is	O
the	O
name	O
given	O
to	O
a	O
number	O
of	O
extensions	O
to	O
his	O
original	O
relational	B-Architecture
model	I-Architecture
(	O
RM	O
)	O
published	O
in	O
1970	O
.	O
</s>
<s>
The	O
overall	O
goal	O
of	O
the	O
RM/T	O
was	O
to	O
define	O
some	O
fundamental	O
semantic	O
units	O
,	O
at	O
"	O
atomic	O
"	O
and	O
"	O
molecular	O
"	O
levels	O
,	O
for	O
data	B-Application
modelling	I-Application
.	O
</s>
<s>
Codd	O
writes	O
:	O
"	O
the	O
result	O
is	O
a	O
model	O
with	O
a	O
richer	O
variety	O
of	O
objects	O
than	O
the	O
original	O
relational	B-Architecture
model	I-Architecture
,	O
additional	O
insert-update-delete	O
rules	O
and	O
some	O
additional	O
operators	O
that	O
make	O
the	O
algebra	O
more	O
powerful.	O
"	O
</s>
<s>
Between	O
1968	O
and	O
1988	O
Codd	O
published	O
over	O
30	O
papers	O
on	O
the	O
relational	B-Architecture
model	I-Architecture
(	O
RM	O
)	O
-	O
the	O
most	O
famous	O
of	O
which	O
is	O
his	O
1970	O
paper	O
.	O
</s>
<s>
Surrogates	O
A	O
surrogate	B-Application
is	O
a	O
unique	O
value	O
assigned	O
to	O
each	O
entity	O
.	O
</s>
<s>
If	O
two	O
relations	O
use	O
the	O
same	O
surrogate	B-Application
value	O
then	O
they	O
represent	O
the	O
same	O
entity	O
in	O
the	O
modelled	O
universe	O
.	O
</s>
<s>
The	O
surrogate	B-Application
value	O
can	O
be	O
any	O
unique	O
string	O
or	O
number	O
but	O
cannot	O
be	O
assigned	O
or	O
changed	O
by	O
the	O
database	O
user	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
SQL	O
SEQUENCE	O
is	O
often	O
used	O
to	O
generate	O
numerical	O
surrogate	B-Application
values	O
.	O
</s>
<s>
This	O
use	O
of	O
surrogate	B-Application
was	O
first	O
introduced	O
in	O
Hall	O
,	O
Owlett	O
and	O
Todd	O
in	O
1976	O
.	O
</s>
<s>
Entities	O
and	O
Nonentities	O
An	O
entity	O
is	O
some	O
thing	O
in	O
the	O
modelled	O
universe	O
and	O
is	O
typically	O
identified	O
by	O
a	O
surrogate	B-Application
.	O
</s>
<s>
A	O
nonentity	O
is	O
some	O
thing	O
that	O
is	O
not	O
an	O
entity	O
and	O
does	O
not	O
have	O
its	O
own	O
identifying	O
surrogate	B-Application
.	O
</s>
<s>
An	O
independent	O
entity	O
has	O
its	O
own	O
surrogate	B-Application
.	O
</s>
<s>
A	O
dependent	O
entity	O
has	O
a	O
surrogate	B-Application
but	O
it	O
belongs	O
to	O
another	O
entity	O
,	O
i.e.	O
</s>
<s>
the	O
surrogate	B-Application
is	O
a	O
foreign	O
key	O
.	O
</s>
<s>
Each	O
E-relation	O
shares	O
its	O
surrogate	B-Application
with	O
the	O
associated	O
P-relations	O
.	O
</s>
<s>
A	O
surrogate	B-Application
value	O
entered	O
into	O
the	O
E-relation	O
table	O
implies	O
the	O
corresponding	O
existence	O
of	O
an	O
entity	O
of	O
that	O
type	O
in	O
the	O
modelled	O
world	O
.	O
</s>
<s>
A	O
P-relation	O
is	O
a	O
relation	O
(	O
table	O
)	O
storing	O
the	O
surrogate	B-Application
and	O
one	O
or	O
more	O
attributes	O
of	O
an	O
entity	O
.	O
</s>
<s>
The	O
surrogate	B-Application
value	O
of	O
a	O
P-relation	O
is	O
that	O
of	O
the	O
corresponding	O
E-relation	O
;	O
it	O
plays	O
the	O
role	O
(	O
K-role	O
)	O
of	O
the	O
primary	O
key	O
for	O
that	O
P-relation	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
P-relation	O
"	O
Employee_Number	O
"	O
is	O
a	O
table	O
with	O
two	O
columns	O
:	O
one	O
containing	O
the	O
surrogate	B-Application
value	O
of	O
the	O
"	O
Employee	O
"	O
E-relation	O
,	O
the	O
other	O
containing	O
the	O
employee	O
number	O
.	O
</s>
<s>
Associative	O
Entity	O
and	O
Nonentity	O
Association	O
An	O
associative	O
entity	O
is	O
an	O
entity	O
that	O
represents	O
an	O
association	O
between	O
two	O
independent	O
entities	O
;	O
the	O
associative	O
entity	O
is	O
an	O
entity	O
in	O
itself	O
because	O
it	O
has	O
a	O
surrogate	B-Application
.	O
</s>
<s>
A	O
nonentity	O
association	O
is	O
similar	O
to	O
an	O
associative	O
entity	O
however	O
it	O
has	O
no	O
surrogate	B-Application
.	O
</s>
<s>
This	O
lack	O
of	O
a	O
surrogate	B-Application
stops	O
the	O
nonentity	O
association	O
from	O
having	O
,	O
for	O
example	O
,	O
any	O
descriptive	O
characteristic	O
entities	O
.	O
</s>
<s>
Date	O
's	O
most	O
recent	O
reflections	O
can	O
be	O
found	O
on	O
the	O
Web	O
at	O
Date	O
(	O
1999	O
)	O
,	O
The	O
Database	O
Relational	B-Architecture
Model	I-Architecture
(	O
2001	O
)	O
and	O
Date	O
on	O
RM/T	O
(	O
2003	O
)	O
.	O
</s>
<s>
RM/T	O
contributed	O
to	O
the	O
body	O
of	O
knowledge	O
called	O
semantic	O
data	B-Application
modeling	I-Application
and	O
semantic	O
object	O
modeling	O
and	O
continues	O
to	O
influence	O
new	O
data	O
modellers	O
.	O
</s>
