<s>
In	O
relational	O
databases	O
a	O
virtual	B-General_Concept
column	I-General_Concept
is	O
a	O
table	O
column	B-Application
whose	O
value	O
is	O
automatically	O
computed	O
using	O
other	O
columns	O
values	O
,	O
or	O
another	O
deterministic	O
expression	O
.	O
</s>
<s>
Virtual	B-General_Concept
columns	I-General_Concept
are	O
defined	O
of	O
SQL:2003	B-Language
as	O
Generated	O
Column	B-Application
,	O
and	O
are	O
only	O
implemented	O
by	O
some	O
DBMSs	O
,	O
like	O
MariaDB	B-Application
,	O
SQL	B-Application
Server	I-Application
,	O
Oracle	B-General_Concept
,	O
PostgreSQL	B-Application
,	O
SQLite	B-Language
and	O
Firebird	B-Application
(	O
database	O
server	O
)	O
(	O
COMPUTED	O
BY	O
syntax	O
)	O
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
virtual	B-General_Concept
columns	I-General_Concept
:	O
</s>
<s>
Virtual	B-General_Concept
columns	I-General_Concept
;	O
</s>
<s>
Virtual	B-General_Concept
columns	I-General_Concept
values	O
are	O
computed	O
on	O
the	O
fly	O
when	O
needed	O
,	O
for	O
example	O
when	O
they	O
are	O
returned	O
by	O
a	O
SELECT	O
statement	O
.	O
</s>
<s>
Persistent	O
column	B-Application
values	O
are	O
computed	O
when	O
a	O
row	B-Application
is	O
inserted	O
in	O
a	O
table	O
,	O
and	O
they	O
are	O
written	O
like	O
all	O
other	O
values	O
.	O
</s>
<s>
Both	O
virtual	O
and	O
persistent	O
columns	O
have	O
advantages	O
and	O
disadvantages	O
:	O
virtual	B-General_Concept
columns	I-General_Concept
do	O
n't	O
consume	O
space	O
on	O
the	O
disk	O
,	O
but	O
they	O
must	O
be	O
computed	O
every	O
time	O
a	O
query	O
refers	O
to	O
them	O
;	O
persistent	O
columns	O
do	O
n't	O
require	O
any	O
CPU	O
time	O
,	O
but	O
they	O
consume	O
disk	O
space	O
.	O
</s>
<s>
However	O
,	O
sometimes	O
a	O
choice	O
is	O
not	O
available	O
,	O
because	O
some	O
DBMS	B-General_Concept
's	O
support	O
only	O
one	O
column	B-Application
type	O
(	O
or	O
neither	O
of	O
them	O
)	O
.	O
</s>
<s>
IBM	B-Application
Db2	I-Application
supports	O
Virtual	B-General_Concept
column	I-General_Concept
of	O
Version	O
8	O
as	O
Generated	O
column	B-Application
.	O
</s>
<s>
MariaDB	B-Application
is	O
a	O
MySQL	B-Application
fork	O
.	O
</s>
<s>
Virtual	B-General_Concept
columns	I-General_Concept
were	O
added	O
in	O
the	O
5.2	O
tree	O
.	O
</s>
<s>
Expressions	O
that	O
can	O
be	O
used	O
to	O
compute	O
the	O
virtual	B-General_Concept
columns	I-General_Concept
have	O
the	O
following	O
limitations	O
:	O
</s>
<s>
They	O
cannot	O
use	O
User	O
Defined	O
Functions	O
or	O
Stored	B-General_Concept
Procedures	I-General_Concept
.	O
</s>
<s>
They	O
cannot	O
include	O
other	O
virtual	B-General_Concept
columns	I-General_Concept
.	O
</s>
<s>
Persistent	O
columns	O
can	O
be	O
indexed	O
and	O
can	O
be	O
part	O
of	O
a	O
foreign	B-Application
key	I-Application
,	O
with	O
a	O
few	O
small	O
limitations	O
concerning	O
constraint	O
enforcement	O
.	O
</s>
<s>
Virtual	B-General_Concept
columns	I-General_Concept
can	O
only	O
be	O
used	O
on	O
tables	O
which	O
use	O
a	O
storage	O
engine	O
which	O
supports	O
them	O
.	O
</s>
<s>
Storage	O
engines	O
supporting	O
virtual	B-General_Concept
columns	I-General_Concept
are	O
:	O
</s>
<s>
MRG_MyISAM	O
tables	O
can	O
be	O
based	O
on	O
MyISAM	B-Language
tables	O
which	O
include	O
persistent	O
columns	O
;	O
but	O
the	O
corresponding	O
MRG_MyISAM	O
column	B-Application
should	O
be	O
defined	O
as	O
a	O
regular	O
column	B-Application
.	O
</s>
<s>
A	O
CREATE	O
TABLE	O
or	O
ALTER	O
TABLE	O
statement	O
can	O
be	O
used	O
to	O
add	O
a	O
virtual	B-General_Concept
column	I-General_Concept
.	O
</s>
<s>
The	O
syntax	O
used	O
to	O
define	O
a	O
virtual	B-General_Concept
column	I-General_Concept
is	O
the	O
following	O
:	O
</s>
<s>
type	O
is	O
the	O
column	B-Application
's	O
data	O
type	O
.	O
</s>
<s>
expression	O
is	O
the	O
SQL	O
expression	O
which	O
returns	O
the	O
column	B-Application
's	O
value	O
for	O
each	O
row	B-Application
.	O
</s>
<s>
text	O
is	O
an	O
optional	O
column	B-Application
comment	O
.	O
</s>
<s>
Support	O
for	O
virtual	B-General_Concept
columns	I-General_Concept
,	O
known	O
in	O
MySQL	B-Application
as	O
generated	O
columns	O
,	O
started	O
becoming	O
available	O
in	O
MySQL	B-Application
5.7	O
.	O
</s>
<s>
Since	O
version	O
11g	O
,	O
Oracle	B-General_Concept
supports	O
virtual	B-General_Concept
columns	I-General_Concept
.	O
</s>
<s>
Microsoft	B-Application
SQL	I-Application
Server	I-Application
supports	O
virtual	B-General_Concept
columns	I-General_Concept
,	O
but	O
they	O
are	O
called	O
Computed	O
Columns	O
.	O
</s>
<s>
SQL	B-Application
Server	I-Application
supports	O
both	O
persisted	O
and	O
non-persisted	O
computed	O
columns	O
.	O
</s>
<s>
Firebird	B-Application
has	O
always	O
supported	O
virtual	B-General_Concept
columns	I-General_Concept
as	O
its	O
precursor	O
InterBase	B-Application
supports	O
it	O
,	O
called	O
Computed	O
Columns	O
.	O
</s>
<s>
Firebird	B-Application
supports	O
virtual	B-General_Concept
columns	I-General_Concept
,	O
not	O
persistent	O
ones	O
and	O
allows	O
for	O
sub-selects	O
,	O
calling	O
built	O
in	O
functions	O
,	O
external	O
functions	O
and	O
stored	O
routines	O
in	O
the	O
virtual	B-General_Concept
column	I-General_Concept
expression	O
.	O
</s>
<s>
Creating	O
a	O
virtual	B-General_Concept
column	I-General_Concept
can	O
be	O
done	O
during	O
table	O
creation	O
and	O
when	O
adding	O
columns	O
to	O
an	O
existing	O
table	O
,	O
the	O
syntax	O
used	O
to	O
define	O
a	O
virtual	B-General_Concept
column	I-General_Concept
is	O
the	O
following	O
:	O
</s>
<s>
Since	O
version	O
12	O
,	O
PostgreSQL	B-Application
supports	O
virtual	B-General_Concept
columns	I-General_Concept
,	O
known	O
as	O
generated	O
columns	O
.	O
</s>
<s>
Since	O
version	O
3.31.0	O
(	O
2020-01-22	O
)	O
,	O
SQLite	B-Language
supports	O
virtual	B-General_Concept
columns	I-General_Concept
,	O
known	O
as	O
generated	O
columns	O
.	O
</s>
