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MySQL 5.5 provides support for server-side prepared
statements. This support takes advantage of the efficient
client/server binary protocol, 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 23.7.7, “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:
You can use it when no programming interface is available to you.
You can use it from any program that enables you to send SQL statements to the server to be executed, such as the mysql client program.
You can use it even if the client is using an old version of the client library. The only requirement is that you be able to connect to a server that is recent enough to support SQL syntax for prepared statements.
SQL syntax for prepared statements is intended to be used for situations such as these:
You want to test how prepared statements work in your application before coding it.
An application has problems executing prepared statements and you want to determine interactively what the problem is.
You want to create a test case that describes a problem you are having with prepared statements, so that you can file a bug report.
You need to use prepared statements but do not have access to a programming API that supports them.
SQL syntax for prepared statements is based on three SQL statements:
PREPARE
prepares a statement for execution (see Section 13.5.1, “PREPARE Statement”).EXECUTE
executes a prepared statement (see Section 13.5.2, “EXECUTE Statement”).DEALLOCATE PREPARE
releases a prepared statement (see Section 13.5.3, “DEALLOCATE PREPARE Statement”).
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 as prepared statements:
ALTER TABLE
ANALYZE TABLE
CACHE INDEX
CALL
CHANGE MASTER
CHECKSUM {TABLE | TABLES}
COMMIT
{CREATE | RENAME | DROP} DATABASE
{CREATE | DROP} INDEX
{CREATE | RENAME | DROP} TABLE
{CREATE | RENAME | DROP} USER
{CREATE | DROP} VIEW
DELETE
DO
FLUSH {TABLE | TABLES | TABLES WITH READ LOCK | HOSTS | PRIVILEGES
| LOGS | STATUS | MASTER | SLAVE | DES_KEY_FILE | USER_RESOURCES}
GRANT
INSERT
INSTALL PLUGIN
KILL
LOAD INDEX INTO CACHE
OPTIMIZE TABLE
REPAIR TABLE
REPLACE
RESET {MASTER | SLAVE | QUERY CACHE}
REVOKE
SELECT
SET
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}
TRUNCATE TABLE
UNINSTALL PLUGIN
UPDATE
Other statements are not supported in MySQL 5.5.
Generally, statements not permitted in SQL prepared statements are also not permitted in stored programs. Exceptions are noted in Section 20.8, “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 13.2.9, “SELECT Statement”.
In prepared CALL
statements used with
PREPARE
and
EXECUTE
, placeholder support for
OUT
and INOUT
parameters is
available beginning with MySQL 5.5. See
Section 13.2.1, “CALL Statement”, for an example and a workaround for earlier
versions. 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).
Prepared statements use the query cache under the conditions described in Section 8.10.3.1, “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 Statement”.