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
t1 in the outer query.
Suppose that table
t1 contains a row where
column1 = 5 and
6; meanwhile, table
t2 contains a
column1 = 5 and
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
not equal to
(5,7)), so the expression as a
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
FROM t2 AS x ... renames
t2. It is
not a column in table
SELECT column1 FROM t1 ... is an outer query
that is farther out.
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 (
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
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,
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
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
HAVINGcondition, but cannot be part of a
JOINcondition, and cannot contain a
OFFSETclause. In addition, the subquery cannot contain any set operations such as
WHEREclause may contain one or more predicates, combined with
AND. If the
WHEREclause contains an
ORclause, it cannot be transformed. At least one of the
WHEREclause predicates must be eligible for transformation, and none of them may reject transformation.
To be eligible for transformation, a
WHEREclause 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.
WHEREclause predicate that contains only inner references is not eligible for transformation, since it can be evaluated before the grouping. A
WHEREclause 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
WHEREclause 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
WHEREclause (and not in the
ORDER BYclause, a
GROUP BYlist, or a
HAVINGclause). Nor can there be any correlated column inside a derived table in the subquery's
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
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 the
SELECTlist element of the subquery, must be at the topmost level, and cannot be part of an expression.
See also Section 22.214.171.124, “Derived Tables”.