<s>
SQL:1999	B-Language
(	O
also	O
called	O
SQL	B-Language
3	I-Language
)	O
was	O
the	O
fourth	O
revision	O
of	O
the	O
SQL	B-Language
database	I-Language
query	B-Language
language	I-Language
.	O
</s>
<s>
It	O
introduced	O
many	O
new	O
features	O
,	O
many	O
of	O
which	O
required	O
clarifications	O
in	O
the	O
subsequent	O
SQL:2003	B-Language
.	O
</s>
<s>
In	O
the	O
meanwhile	O
SQL:1999	B-Language
is	O
deprecated	O
.	O
</s>
<s>
The	O
first	O
installment	O
of	O
SQL:1999	B-Language
had	O
five	O
parts	O
:	O
</s>
<s>
Three	O
more	O
parts	O
,	O
also	O
considered	O
part	O
of	O
SQL:1999	B-Language
were	O
published	O
subsequently	O
:	O
</s>
<s>
The	O
SQL:1999	B-Language
standard	O
calls	O
for	O
a	O
Boolean	O
type	O
,	O
but	O
many	O
commercial	O
SQL	B-Language
servers	I-Language
(	O
Oracle	B-General_Concept
Database	I-General_Concept
,	O
IBM	B-Application
Db2	I-Application
)	O
do	O
not	O
support	O
it	O
as	O
a	O
column	O
type	O
,	O
variable	O
type	O
or	O
allow	O
it	O
in	O
the	O
results	O
set	O
.	O
</s>
<s>
Microsoft	B-Application
SQL	I-Application
Server	I-Application
is	O
one	O
of	O
the	O
few	O
database	O
systems	O
that	O
properly	O
supports	O
BOOLEAN	O
values	O
using	O
its	O
"	O
BIT	O
"	O
data	O
type	O
.	O
</s>
<s>
MySQL	B-Application
interprets	O
"	O
BOOLEAN	O
"	O
as	O
a	O
synonym	O
for	O
TINYINT	O
(	O
8-bit	O
signed	O
integer	O
)	O
.	O
</s>
<s>
PostgreSQL	B-Application
provides	O
a	O
standard	O
conforming	O
Boolean	O
type	O
.	O
</s>
<s>
Sometimes	O
called	O
just	O
distinct	B-Language
types	I-Language
,	O
these	O
were	O
introduced	O
as	O
an	O
optional	O
feature	O
(	O
S011	O
)	O
to	O
allow	O
existing	O
atomic	O
types	O
to	O
be	O
extended	O
with	O
a	O
distinctive	O
meaning	O
to	O
create	O
a	O
new	O
type	O
and	O
thereby	O
enabling	O
the	O
type	O
checking	O
mechanism	O
to	O
detect	O
some	O
logical	O
errors	O
,	O
e.g.	O
</s>
<s>
The	O
SQL	B-Language
distinct	B-Language
types	I-Language
use	O
name	O
equivalence	O
not	O
structural	O
equivalence	O
like	O
typedefs	B-Language
in	O
C	O
.	O
It	O
's	O
still	O
possible	O
to	O
perform	O
compatible	O
operations	O
on	O
(	O
columns	O
or	O
data	O
)	O
of	O
distinct	B-Language
types	I-Language
by	O
using	O
an	O
explicit	O
type	O
CAST	O
.	O
</s>
<s>
Few	O
SQL	B-Language
systems	O
support	O
these	O
.	O
</s>
<s>
IBM	B-Application
Db2	I-Application
is	O
one	O
those	O
supporting	O
them	O
.	O
</s>
<s>
Oracle	B-General_Concept
database	I-General_Concept
did	O
not	O
support	O
them	O
as	O
of	O
2012	O
,	O
recommending	O
instead	O
to	O
emulate	O
them	O
by	O
a	O
one-place	O
structured	B-Language
type	I-Language
.	O
</s>
<s>
These	O
are	O
the	O
backbone	O
of	O
the	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
database	I-General_Concept
extension	O
in	O
SQL:1999	B-Language
.	O
</s>
<s>
They	O
are	O
analogous	O
to	O
classes	O
in	O
objected-oriented	B-Language
programming	I-Language
languages	I-Language
.	O
</s>
<s>
SQL:1999	B-Language
allows	O
only	O
single	O
inheritance	O
.	O
</s>
<s>
SQL:1999	B-Language
added	O
a	O
WITH	O
 [ RECURSIVE ] 	O
construct	O
allowing	O
recursive	O
queries	O
,	O
like	O
transitive	O
closure	O
,	O
to	O
be	O
specified	O
in	O
the	O
query	B-Language
language	I-Language
itself	O
;	O
see	O
common	O
table	O
expressions	O
.	O
</s>
<s>
SQL:1999	B-Language
introduced	O
the	O
UNNEST	O
keyword	O
.	O
</s>
