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C.4 Restrictions on Subqueries

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  • 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:

    1. DELETE FROM t WHERE ... (SELECT ... FROM t ...);
    2. UPDATE t ... WHERE col = (SELECT ... FROM t ...);
    3. {INSERT|REPLACE} INTO t (SELECT ... FROM t ...);

    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:

    1. UPDATE t ... WHERE col = (SELECT * FROM (SELECT ... FROM t...) AS dt ...);

    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:

    • For expr [NOT] IN subquery, expr 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} subquery, expr 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:

    1. (expr_1, ..., expr_n) [NOT] IN table_subquery

    But this is not supported:

    1. (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 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:

    1. mysql> SELECT * FROM t1
    2.        WHERE s1 IN (SELECT s2 FROM t2 ORDER BY s1 LIMIT 1);
    3. ERROR 1235 (42000): This version of MySQL doesn't yet support
    4. '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:

    1. SELECT ... WHERE x IN (SELECT f() ...);

    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.


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Document created the 26/06/2006, last modified the 26/10/2018
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