13.2.10.7 Correlated Subqueries
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.
For subqueries in HAVING
or ORDER BY
clauses, MySQL also looks for column names in the outer select list.
For certain cases, a correlated subquery is optimized. For example:
val IN (SELECT key_val FROM tbl_name WHERE correlated_condition)
Otherwise, they are inefficient and likely to be slow. Rewriting the query as a join might improve performance.
Aggregate functions in correlated subqueries may contain outer references, provided the function contains nothing but outer references, and provided the function is not contained in another function or expression.