<s>
In	O
a	O
SQL	B-Language
database	I-Language
query	B-Language
,	O
a	O
correlated	B-Language
subquery	I-Language
(	O
also	O
known	O
as	O
a	O
synchronized	O
subquery	O
)	O
is	O
a	O
subquery	O
(	O
a	O
query	B-Language
nested	O
inside	O
another	O
query	B-Language
)	O
that	O
uses	O
values	O
from	O
the	O
outer	O
query	B-Language
.	O
</s>
<s>
Because	O
the	O
subquery	O
may	O
be	O
evaluated	O
once	O
for	O
each	O
row	O
processed	O
by	O
the	O
outer	O
query	B-Language
,	O
it	O
can	O
be	O
slow	O
.	O
</s>
<s>
Here	O
is	O
an	O
example	O
for	O
a	O
typical	O
correlated	B-Language
subquery	I-Language
.	O
</s>
<s>
In	O
the	O
above	O
nested	O
query	B-Language
the	O
inner	O
query	B-Language
has	O
to	O
be	O
re-executed	O
for	O
each	O
employee	O
.	O
</s>
<s>
(	O
A	O
sufficiently	O
smart	O
implementation	O
may	O
cache	O
the	O
inner	O
query	B-Language
's	O
result	O
on	O
a	O
department-by-department	O
basis	O
,	O
but	O
even	O
in	O
the	O
best	O
case	O
the	O
inner	O
query	B-Language
must	O
be	O
executed	O
once	O
per	O
department	O
.	O
)	O
</s>
<s>
Correlated	B-Language
subqueries	I-Language
may	O
appear	O
elsewhere	O
besides	O
the	O
WHERE	B-Language
clause	I-Language
;	O
for	O
example	O
,	O
this	O
query	B-Language
uses	O
a	O
correlated	B-Language
subquery	I-Language
in	O
the	O
SELECT	B-Language
clause	I-Language
to	O
print	O
the	O
entire	O
list	O
of	O
employees	O
alongside	O
the	O
average	O
salary	O
for	O
each	O
employee	O
's	O
department	O
.	O
</s>
<s>
Again	O
,	O
because	O
the	O
subquery	O
is	O
correlated	O
with	O
a	O
column	O
of	O
the	O
outer	O
query	B-Language
,	O
it	O
must	O
be	O
re-executed	O
for	O
each	O
row	O
of	O
the	O
result	O
.	O
</s>
<s>
It	O
is	O
generally	O
meaningless	O
to	O
have	O
a	O
correlated	B-Language
subquery	I-Language
in	O
the	O
FROM	O
clause	O
because	O
the	O
table	O
in	O
the	O
FROM	O
clause	O
is	O
needed	O
to	O
evaluate	O
the	O
outer	O
query	B-Language
,	O
but	O
the	O
correlated	B-Language
subquery	I-Language
in	O
the	O
FROM	O
clause	O
ca	O
n't	O
be	O
evaluated	O
before	O
the	O
outer	O
query	B-Language
is	O
evaluated	O
,	O
causing	O
a	O
chicken-and-egg	O
problem	O
.	O
</s>
<s>
Specifically	O
,	O
MariaDB	B-Application
lists	O
this	O
as	O
a	O
limitation	O
in	O
its	O
documentation	O
.	O
</s>
<s>
However	O
,	O
in	O
some	O
database	O
systems	O
,	O
it	O
is	O
allowed	O
to	O
use	O
correlated	B-Language
subqueries	I-Language
while	O
joining	O
in	O
the	O
FROM	O
clause	O
,	O
referencing	O
the	O
tables	O
listed	O
before	O
the	O
join	O
using	O
a	O
specified	O
keyword	O
,	O
producing	O
a	O
number	O
of	O
rows	O
in	O
the	O
correlated	B-Language
subquery	I-Language
and	O
joining	O
it	O
to	O
the	O
table	O
on	O
the	O
left	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
PostgreSQL	B-Application
,	O
adding	O
the	O
keyword	O
LATERAL	O
before	O
the	O
right-hand	O
subquery	O
,	O
or	O
in	O
Microsoft	B-Application
SQL	I-Application
Server	I-Application
,	O
using	O
the	O
keyword	O
CROSS	O
APPLY	O
or	O
OUTER	O
APPLY	O
instead	O
of	O
JOIN	O
achieves	O
the	O
effect	O
.	O
</s>
<s>
Because	O
this	O
is	O
a	O
customized	O
approach	O
,	O
existing	O
database	O
systems	O
cannot	O
flatten	O
arbitrary	O
correlated	B-Language
subqueries	I-Language
by	O
following	O
certain	O
general	O
rules	O
.	O
</s>
<s>
A	O
general	O
computational	O
approach	O
is	O
to	O
directly	O
execute	O
the	O
nested	O
loop	O
by	O
iterating	O
all	O
tuples	O
of	O
the	O
correlated	O
columns	O
from	O
the	O
outer	O
query	B-Language
block	O
and	O
executing	O
the	O
subquery	O
as	O
many	O
times	O
as	O
the	O
number	O
of	O
outer-loop	O
tuples	O
.	O
</s>
