13.2.8.7. 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 subquery 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.
Correlated subqueries cannot refer to the results of aggregate
functions from the outer query.
