<s>
In	O
temporal	O
databases	O
,	O
valid	B-General_Concept
time	I-General_Concept
(	O
VT	O
)	O
is	O
the	O
time	O
period	O
during	O
which	O
a	O
database	O
fact	O
is	O
valid	O
in	O
the	O
modeled	O
reality	O
.	O
</s>
<s>
As	O
of	O
December	O
2011	O
,	O
ISO/IEC	O
9075	O
,	O
Database	O
Language	O
SQL:2011	B-Language
Part	O
2	O
:	O
SQL/Foundation	O
included	O
clauses	O
in	O
table	O
definitions	O
to	O
define	O
"	O
application-time	O
period	O
tables	O
"	O
(	O
that	O
is	O
,	O
valid-time	B-General_Concept
tables	O
)	O
.	O
</s>
<s>
Valid	B-General_Concept
time	I-General_Concept
was	O
coined	O
by	O
Richard	O
T	O
.	O
Snodgrass	O
and	O
his	O
doctoral	O
student	O
.	O
</s>
<s>
In	O
a	O
database	O
table	O
valid	B-General_Concept
time	I-General_Concept
is	O
often	O
represented	O
by	O
two	O
extra	O
table-columns	O
StartVT	O
and	O
EndVT	O
.	O
</s>
<s>
Valid	B-General_Concept
time	I-General_Concept
is	O
the	O
time	O
for	O
which	O
a	O
fact	O
is	O
true	O
in	O
the	O
real	O
world	O
.	O
</s>
