13.5 SQL Syntax for Prepared Statements

13.5.1 PREPARE Syntax
13.5.2 EXECUTE Syntax
13.5.3 DEALLOCATE PREPARE Syntax
13.5.4 Automatic Prepared Statement Repreparation

MySQL 5.1 provides support for server-side prepared statements. This support takes advantage of the efficient client/server binary protocol implemented in MySQL 4.1, provided that you use an appropriate client programming interface. Candidate interfaces include the MySQL C API client library (for C programs), MySQL Connector/J (for Java programs), and MySQL Connector/Net. For example, the C API provides a set of function calls that make up its prepared statement API. See Section 21.8.8, “C API Prepared Statements”. Other language interfaces can provide support for prepared statements that use the binary protocol by linking in the C client library, one example being the mysqli extension, available in PHP 5.0 and later.

An alternative SQL interface to prepared statements is available. This interface is not as efficient as using the binary protocol through a prepared statement API, but requires no programming because it is available directly at the SQL level:

SQL syntax for prepared statements is intended to be used for situations such as these:

SQL syntax for prepared statements is based on three SQL statements:

The following examples show two equivalent ways of preparing a statement that computes the hypotenuse of a triangle given the lengths of the two sides.

The first example shows how to create a prepared statement by using a string literal to supply the text of the statement:

mysql> PREPARE stmt1 FROM 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> SET @a = 3;
mysql> SET @b = 4;
mysql> EXECUTE stmt1 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|          5 |
+------------+
mysql> DEALLOCATE PREPARE stmt1;

The second example is similar, but supplies the text of the statement as a user variable:

mysql> SET @s = 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> PREPARE stmt2 FROM @s;
mysql> SET @a = 6;
mysql> SET @b = 8;
mysql> EXECUTE stmt2 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|         10 |
+------------+
mysql> DEALLOCATE PREPARE stmt2;

Here is an additional example that demonstrates how to choose the table on which to perform a query at runtime, by storing the name of the table as a user variable:

mysql> USE test;
mysql> CREATE TABLE t1 (a INT NOT NULL);
mysql> INSERT INTO t1 VALUES (4), (8), (11), (32), (80);

mysql> SET @table = 't1';
mysql> SET @s = CONCAT('SELECT * FROM ', @table);

mysql> PREPARE stmt3 FROM @s;
mysql> EXECUTE stmt3;
+----+
| a  |
+----+
|  4 |
|  8 |
| 11 |
| 32 |
| 80 |
+----+

mysql> DEALLOCATE PREPARE stmt3;

A prepared statement is specific to the session in which it was created. If you terminate a session without deallocating a previously prepared statement, the server deallocates it automatically.

A prepared statement is also global to the session. If you create a prepared statement within a stored routine, it is not deallocated when the stored routine ends.

To guard against too many prepared statements being created simultaneously, set the max_prepared_stmt_count system variable. To prevent the use of prepared statements, set the value to 0.

The following SQL statements can be used in prepared statements:

ALTER TABLE
CALL
COMMIT
{CREATE | DROP} INDEX
{CREATE | DROP} TABLE
{CREATE | DROP} VIEW
DELETE
DO
INSERT
RENAME TABLE
REPLACE
SELECT
SET
SHOW (most variants)
TRUNCATE TABLE
UPDATE

As of MySQL 5.1.10, the following additional statements are supported:

ANALYZE TABLE
OPTIMIZE TABLE
REPAIR TABLE

As of MySQL 5.1.12, the following additional statements are supported:

CACHE INDEX
CHANGE MASTER
CHECKSUM {TABLE | TABLES}
{CREATE | RENAME | DROP} DATABASE
{CREATE | RENAME | DROP} USER
FLUSH {TABLE | TABLES | TABLES WITH READ LOCK | HOSTS | PRIVILEGES
  | LOGS | STATUS | MASTER | SLAVE | DES_KEY_FILE | USER_RESOURCES}
GRANT
INSTALL PLUGIN
KILL
LOAD INDEX INTO CACHE
RESET {MASTER | SLAVE | QUERY CACHE}
REVOKE
SHOW {AUTHORS | CONTRIBUTORS | WARNINGS | ERRORS}
SHOW BINLOG EVENTS
SHOW CREATE {PROCEDURE | FUNCTION | EVENT | TABLE | VIEW}
SHOW {MASTER | BINARY} LOGS
SHOW {MASTER | SLAVE} STATUS
SLAVE {START | STOP}
UNINSTALL PLUGIN

Other statements are not supported in MySQL 5.1.

Generally, statements not permitted in SQL prepared statements are also not permitted in stored programs. Exceptions are noted in Section E.1, “Restrictions on Stored Programs”.

Metadata changes to tables or views referred to by prepared statements are detected and cause automatic repreparation of the statement when it is next executed. For more information, see Section 13.5.4, “Automatic Prepared Statement Repreparation”.

Placeholders can be used for the arguments of the LIMIT clause when using prepared statements. See Section 13.2.8, “SELECT Syntax”.

In prepared CALL statements used with PREPARE and EXECUTE, placeholder support for OUT and INOUT parameters is not available in MySQL 5.1. See Section 13.2.1, “CALL Syntax”, for an example and a workaround. Placeholders can be used for IN parameters regardless of version.

SQL syntax for prepared statements cannot be used in nested fashion. That is, a statement passed to PREPARE cannot itself be a PREPARE, EXECUTE, or DEALLOCATE PREPARE statement.

SQL syntax for prepared statements is distinct from using prepared statement API calls. For example, you cannot use the mysql_stmt_prepare() C API function to prepare a PREPARE, EXECUTE, or DEALLOCATE PREPARE statement.

SQL syntax for prepared statements can be used within stored procedures, but not in stored functions or triggers. However, a cursor cannot be used for a dynamic statement that is prepared and executed with PREPARE and EXECUTE. The statement for a cursor is checked at cursor creation time, so the statement cannot be dynamic.

SQL syntax for prepared statements does not support multi-statements (that is, multiple statements within a single string separated by ; characters).

Before MySQL 5.1.17, prepared statements do not use the query cache. As of 5.1.17, prepared statements use the query cache under the conditions described in Section 8.6.3.1, “How the Query Cache Operates”.

To write C programs that use the CALL SQL statement to execute stored procedures that contain prepared statements, the CLIENT_MULTI_RESULTS flag must be enabled. This is because each CALL returns a result to indicate the call status, in addition to any result sets that might be returned by statements executed within the procedure.

CLIENT_MULTI_RESULTS can be enabled when you call mysql_real_connect(), either explicitly by passing the CLIENT_MULTI_RESULTS flag itself, or implicitly by passing CLIENT_MULTI_STATEMENTS (which also enables CLIENT_MULTI_RESULTS). For additional information, see Section 13.2.1, “CALL Syntax”.