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.
For subqueries in
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.