In general, you cannot modify a table and select from the same table in a subquery. For example, this limitation applies to statements of the following forms:
DELETE FROM t WHERE ... (SELECT ... FROM t ...);
UPDATE t ... WHERE col = (SELECT ... FROM t ...);
{INSERT|REPLACE} INTO t (SELECT ... FROM t ...);
Exception: The preceding prohibition does not apply if you are
using a subquery for the modified table in the
FROM clause. Example:
UPDATE t ... WHERE col = (SELECT * FROM (SELECT ... FROM t...) AS _t ...);
Here the result from the subquery in the
FROM clause is stored as a temporary table,
so the relevant rows in t have already been
selected by the time the update to t takes
place.
Row comparison operations are only partially supported:
For expr [NOT] IN
subqueryexpr can be an
n-tuple (specified using row
constructor syntax) and the subquery can return rows of
n-tuples. The permitted syntax
is therefore more specifically expressed as
row_constructor [NOT]
IN table_subquery
For expr
op {ALL|ANY|SOME}
subqueryexpr must be a scalar value and
the subquery must be a column subquery; it cannot return
multiple-column rows.
In other words, for a subquery that returns rows of
n-tuples, this is supported:
(expr_1, ...,expr_n) [NOT] INtable_subquery
But this is not supported:
(expr_1, ...,expr_n)op{ALL|ANY|SOME}subquery
The reason for supporting row comparisons for
IN but not for the others is that
IN is implemented by rewriting it as a
sequence of =
comparisons and AND operations.
This approach cannot be used for ALL,
ANY, or SOME.
Subqueries in the FROM clause cannot be
correlated subqueries. They are materialized in whole
(evaluated to produce a result set) during query execution, so
they cannot be evaluated per row of the outer query. The
optimizer delays materialization until the result is needed,
which may permit materialization to be avoided. See
Optimizing Derived Tables and View References.
MySQL does not support LIMIT in subqueries
for certain subquery operators:
mysql>SELECT * FROM t1->WHERE s1 IN (SELECT s2 FROM t2 ORDER BY s1 LIMIT 1);ERROR 1235 (42000): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
MySQL permits a subquery to refer to a stored function that
has data-modifying side effects such as inserting rows into a
table. For example, if f() inserts rows,
the following query can modify data:
SELECT ... WHERE x IN (SELECT f() ...);
This behavior is an extension to the SQL standard. In MySQL,
it can produce indeterminate results because
f() might be executed a different number of
times for different executions of a given query depending on
how the optimizer chooses to handle it.
For statement-based or mixed-format replication, one implication of this indeterminism is that such a query can produce different results on the master and its slaves.