A correlated subquery is a subquery that contains a reference to a table that also appears in the outer query. For example:
SELECT * FROM t1
WHERE column1 = ANY (SELECT column1 FROM t2
WHERE t2.column2 = t1.column2);
Notice that the subquery contains a reference to a column of
t1
, even though the subquery's
FROM
clause does not mention a table
t1
. So, MySQL looks outside the subquery, and
finds t1
in the outer query.
Suppose that table t1
contains a row where
column1 = 5
and column2 =
6
; meanwhile, table t2
contains a
row where column1 = 5
and column2 =
7
. The simple expression ... WHERE column1 =
ANY (SELECT column1 FROM t2)
would be
TRUE
, but in this example, the
WHERE
clause within the subquery is
FALSE
(because (5,6)
is
not equal to (5,7)
), so the expression as a
whole is FALSE
.
Scoping rule: MySQL evaluates from inside to outside. For example:
SELECT column1 FROM t1 AS x
WHERE x.column1 = (SELECT column1 FROM t2 AS x
WHERE x.column1 = (SELECT column1 FROM t3
WHERE x.column2 = t3.column1));
In this statement, x.column2
must be a column
in table t2
because SELECT column1
FROM t2 AS x ...
renames t2
. It is
not a column in table t1
because
SELECT column1 FROM t1 ...
is an outer query
that is farther out.
The optimizer can transform a correlated scalar subquery to a
derived table when the
subquery_to_derived
flag of
the optimizer_switch
variable
is enabled. Consider the query shown here:
SELECT * FROM t1
WHERE ( SELECT a FROM t2
WHERE t2.a=t1.a ) > 0;
To avoid materializing several times for a given derived table,
we can instead materialize—once—a derived table
which adds a grouping on the join column from the table
referenced in the inner query (t2.a
) and then
an outer join on the lifted predicate (t1.a =
derived.a
) in order to select the correct group to
match up with the outer row. (If the subquery already has an
explicit grouping, the extra grouping is added to the end of the
grouping list.) The query previously shown can thus be rewritten
like this:
SELECT t1.* FROM t1
LEFT OUTER JOIN
(SELECT a, COUNT(*) AS ct FROM t2 GROUP BY a) AS derived
ON t1.a = derived.a
AND
REJECT_IF(
(ct > 1),
"ERROR 1242 (21000): Subquery returns more than 1 row"
)
WHERE derived.a > 0;
In the rewritten query, REJECT_IF()
represents an internal function which tests a given condition
(here, the comparison ct > 1
) and raises a
given error (in this case,
ER_SUBQUERY_NO_1_ROW
) if the
condition is true. This reflects the cardinality check that the
optimizer performs as part of evaluating the
JOIN
or WHERE
clause,
prior to evaluating any lifted predicate, which is done only if
the subquery does not return more than one row.
This type of transformation can be performed, provided the following conditions are met:
The subquery can be part of a
SELECT
list,WHERE
condition, orHAVING
condition, but cannot be part of aJOIN
condition, and cannot contain anOFFSET
clause. The subquery may containLIMIT 1
but no otherLIMIT
clause; it must use a literal1
, and no other value, placeholder (?
), or variable. In addition, the subquery cannot contain any set operations such asUNION
.The
WHERE
clause may contain one or more predicates, combined withAND
. If theWHERE
clause contains anOR
clause, it cannot be transformed. At least one of theWHERE
clause predicates must be eligible for transformation, and none of them may reject transformation.To be eligible for transformation, a
WHERE
clause predicate must be an equality predicate in which each operand should be a simple column reference. No other predicates—including other comparison predicates—are eligible for transformation. The predicate must employ the equality operator=
for making the comparison; the null-safe<=>
operator is not supported in this context.A
WHERE
clause predicate that contains only inner references is not eligible for transformation, since it can be evaluated before the grouping. AWHERE
clause predicate that contains only outer references is eligible for transformation, even though it can be lifted up to the outer query block. This is made possible by adding a cardinality check without grouping in the derived table.To be eligible, a
WHERE
clause predicate must have one operand that contains only inner references and one operand that contains only outer references. If the predicate is not eligible due to this rule, transformation of the query is rejected.A correlated column can be present only in the subquery's
WHERE
clause (and not in theSELECT
list, aJOIN
orORDER BY
clause, aGROUP BY
list, or aHAVING
clause). Nor can there be any correlated column inside a derived table in the subquery'sFROM
list.A correlated column can not be contained in an aggregate function's list of arguments.
A correlated column must be resolved in the query block directly containing the subquery being considered for transformation.
A correlated column cannot be present in a nested scalar subquery in the
WHERE
clause.The subquery cannot contain any window functions, and must not contain any aggregate function which aggregates in a query block outer to the subquery. A
COUNT()
aggregate function, if contained in theSELECT
list element of the subquery, must be at the topmost level, and cannot be part of an expression.
See also Section 15.2.15.8, “Derived Tables”.