<s>
The	O
DUAL	B-Application
table	I-Application
is	O
a	O
special	O
one-row	O
,	O
one-column	O
table	B-Application
present	O
by	O
default	O
in	O
Oracle	B-General_Concept
and	O
other	O
database	B-Application
installations	O
.	O
</s>
<s>
In	O
Oracle	B-General_Concept
,	O
the	O
table	B-Application
has	O
a	O
single	O
VARCHAR2(1 )	O
column	O
called	O
DUMMY	O
that	O
has	O
a	O
value	O
of	O
'	O
X	O
 '	O
.	O
</s>
<s>
Oracle	B-General_Concept
's	O
SQL	B-Language
syntax	O
requires	O
the	O
FROM	B-Language
clause	O
but	O
some	O
queries	O
do	O
n't	O
require	O
any	O
tables	O
-	O
DUAL	O
can	O
be	O
used	O
in	O
these	O
cases	O
.	O
</s>
<s>
I	O
created	O
the	O
DUAL	B-Application
table	I-Application
as	O
an	O
underlying	O
object	O
in	O
the	O
Oracle	B-General_Concept
Data	O
Dictionary	O
.	O
</s>
<s>
The	O
idea	O
was	O
that	O
you	O
could	O
do	O
a	O
JOIN	O
to	O
the	O
DUAL	B-Application
table	I-Application
and	O
create	O
two	O
rows	O
in	O
the	O
result	O
for	O
every	O
one	O
row	O
in	O
your	O
table	B-Application
.	O
</s>
<s>
The	O
name	O
,	O
DUAL	O
,	O
seemed	O
apt	O
for	O
the	O
process	O
of	O
creating	O
a	O
pair	O
of	O
rows	O
from	B-Language
just	O
one	O
.	O
</s>
<s>
Beginning	O
with	O
10g	O
Release	O
1	O
,	O
Oracle	B-General_Concept
no	O
longer	O
performs	O
physical	O
or	O
logical	O
I/O	O
on	O
the	O
DUAL	B-Application
table	I-Application
,	O
though	O
the	O
table	B-Application
still	O
exists	O
.	O
</s>
<s>
DUAL	O
is	O
readily	O
available	O
for	O
all	O
authorized	O
users	O
in	O
a	O
SQL	B-Language
database	I-Language
.	O
</s>
<s>
Several	O
other	O
databases	O
(	O
including	O
Microsoft	B-Application
SQL	I-Application
Server	I-Application
,	O
MySQL	B-Application
,	O
PostgreSQL	B-Application
,	O
SQLite	B-Language
,	O
and	O
Teradata	O
)	O
enable	O
one	O
to	O
omit	O
the	O
FROM	B-Language
clause	O
entirely	O
if	O
no	O
table	B-Application
is	O
needed	O
.	O
</s>
<s>
This	O
avoids	O
the	O
need	O
for	O
any	O
dummy	O
table	B-Application
.	O
</s>
<s>
ClickHouse	B-Language
has	O
a	O
one-row	O
system	O
table	B-Application
system.one	O
with	O
a	O
single	O
column	O
named	O
"	O
dummy	O
"	O
of	O
type	O
UInt8	O
and	O
value	O
0	O
.	O
</s>
<s>
This	O
table	B-Application
is	O
implicitly	O
used	O
when	O
no	O
table	B-Application
is	O
specified	O
in	O
the	O
SELECT	O
query	O
.	O
</s>
<s>
Firebird	B-Application
has	O
a	O
one-row	O
system	O
table	B-Application
RDB$DATABASE	O
that	O
is	O
used	O
in	O
the	O
same	O
way	O
as	O
Oracle	B-General_Concept
's	O
DUAL	O
,	O
although	O
it	O
also	O
has	O
a	O
meaning	O
of	O
its	O
own	O
.	O
</s>
<s>
IBM	B-Application
Db2	I-Application
has	O
a	O
view	O
that	O
resolves	O
DUAL	O
when	O
using	O
Oracle	B-General_Concept
Compatibility	O
.	O
</s>
<s>
It	O
also	O
has	O
a	O
table	B-Application
called	O
sysibm.sysdummy1	O
that	O
has	O
similar	O
properties	O
to	O
the	O
Oracle	B-General_Concept
DUAL	O
one	O
.	O
</s>
<s>
Informix	B-Application
:	O
Informix	B-Application
version	O
11.50	O
and	O
later	O
has	O
a	O
table	B-Application
named	O
with	O
the	O
same	O
functionality	O
but	O
a	O
more	O
verbose	O
name	O
.	O
</s>
<s>
You	O
can	O
use	O
to	O
create	O
a	O
name	O
in	O
the	O
current	O
database	B-Application
with	O
the	O
same	O
functionality	O
.	O
</s>
<s>
Microsoft	B-Application
SQL	I-Application
Server	I-Application
:	O
SQL	B-Language
Server	O
does	O
not	O
require	O
a	O
dummy	O
table	B-Application
.	O
</s>
<s>
Queries	O
like	O
'	O
select	O
1	O
+	O
1	O
 '	O
can	O
be	O
run	O
without	O
a	O
"	O
from	B-Language
"	O
clause/table	O
name	O
.	O
</s>
<s>
MySQL	B-Application
allows	O
DUAL	O
to	O
be	O
specified	O
as	O
a	O
table	B-Application
in	O
queries	O
that	O
do	O
not	O
need	O
data	O
from	B-Language
any	O
tables	O
.	O
</s>
<s>
PostgreSQL	B-Application
:	O
A	O
DUAL-view	O
can	O
be	O
added	O
to	O
ease	O
porting	O
from	B-Language
Oracle	B-General_Concept
.	O
</s>
<s>
Snowflake	B-General_Concept
:	O
DUAL	O
is	O
supported	O
,	O
but	O
not	O
explicitly	O
documented	O
.	O
</s>
<s>
SQLite	B-Language
:	O
A	O
VIEW	O
named	O
"	O
dual	O
"	O
that	O
works	O
the	O
same	O
as	O
the	O
Oracle	B-General_Concept
"	O
dual	O
"	O
table	B-Application
can	O
be	O
created	O
as	O
follows	O
:	O
</s>
<s>
SAP	B-Application
HANA	I-Application
has	O
a	O
table	B-Application
called	O
DUMMY	O
that	O
works	O
the	O
same	O
as	O
the	O
Oracle	B-General_Concept
"	O
dual	O
"	O
table	B-Application
.	O
</s>
<s>
Teradata	O
database	B-Application
does	O
not	O
require	O
a	O
dummy	O
table	B-Application
.	O
</s>
<s>
Queries	O
like	O
'	O
select	O
1	O
+	O
1	O
 '	O
can	O
be	O
run	O
without	O
a	O
"	O
from	B-Language
"	O
clause/table	O
name	O
.	O
</s>
<s>
Vertica	O
has	O
support	O
for	O
a	O
DUAL	B-Application
table	I-Application
in	O
their	O
official	O
documentation	O
.	O
</s>
