C.4 Restrictions on Subqueries

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:

Exception: The preceding prohibition does not apply if for the modified table you are using a derived table and that derived table is materialized rather than merged into the outer query. (See Section 8.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.) Example:

Here the result from the derived table is materialized as a temporary table, so the relevant rows in t have already been selected by the time the update to t takes place.

In general, you may be able to influence the optimizer to materialize a derived table by adding a NO_MERGE optimizer hint. See Section 8.9.3, “Optimizer Hints”.

Row comparison operations are only partially supported:

In other words, for a subquery that returns rows of n-tuples, this is supported:

But this is not supported:

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 Section 8.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”.

MySQL does not support LIMIT in subqueries for certain subquery operators:

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:

This behavior is an extension to the SQL standard. In MySQL, it can produce nondeterministic 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.