Applications that use the MySQL C API define query attributes by calling themysql_bind_param()function. Seemysql_bind_param(). Other MySQL connectors may also provide query-attribute support. See the documentation for individual connectors.

Themysqlclient has aquery_attributescommand that enables defining up to 32 pairs of attribute names and values. SeeSection 4.5.1.2, “mysql Client Commands”.

Query attribute names are transmitted using the character set indicated by thecharacter_set_clientsystem variable.

To access query attributes within SQL statements for which attributes have been defined, install thequery_attributescomponent as described inPrerequisites for Using Query Attributes. The component implements amysql_query_attribute_string()loadable function that takes an attribute name argument and returns the attribute value as a string, orNULLif the attribute does not exist. SeeQuery Attribute Loadable Functions.

The following examples use themysqlclientquery_attributescommand to define attribute name/value pairs, and themysql_query_attribute_string()function to access attribute values by name.

This example defines two attributes namedn1andn2. The firstSELECTshows how to retrieve those attributes, and also demonstrates that retrieving a nonexistent attribute (n3) returnsNULL. The secondSELECTshows that attributes do not persist across statements.

mysql> query_attributes n1 v1 n2 v2; mysql> SELECT mysql_query_attribute_string('n1') AS 'attr 1', mysql_query_attribute_string('n2') AS 'attr 2', mysql_query_attribute_string('n3') AS 'attr 3'; +--------+--------+--------+ | attr 1 | attr 2 | attr 3 | +--------+--------+--------+ | v1 | v2 | NULL | +--------+--------+--------+ mysql> SELECT mysql_query_attribute_string('n1') AS 'attr 1', mysql_query_attribute_string('n2') AS 'attr 2'; +--------+--------+ | attr 1 | attr 2 | +--------+--------+ | NULL | NULL | +--------+--------+

As shown by the secondSELECTstatement, attributes defined prior to a given statement are available only to that statement and are cleared after the statement executes. To use an attribute value across multiple statements, assign it to a variable. The following example shows how to do this, and illustrates that attribute values are available in subsequent statements by means of the variables, but not by callingmysql_query_attribute_string():

mysql> query_attributes n1 v1 n2 v2; mysql> SET @attr1 = mysql_query_attribute_string('n1'), @attr2 = mysql_query_attribute_string('n2'); mysql> SELECT @attr1, mysql_query_attribute_string('n1') AS 'attr 1', @attr2, mysql_query_attribute_string('n2') AS 'attr 2'; +--------+--------+--------+--------+ | @attr1 | attr 1 | @attr2 | attr 2 | +--------+--------+--------+--------+ | v1 | NULL | v2 | NULL | +--------+--------+--------+--------+

Attributes can also be saved for later use by storing them in a table:

mysql> CREATE TABLE t1 (c1 CHAR(20), c2 CHAR(20)); mysql> query_attributes n1 v1 n2 v2; mysql> INSERT INTO t1 (c1, c2) VALUES( mysql_query_attribute_string('n1'), mysql_query_attribute_string('n2') ); mysql> SELECT * FROM t1; +------+------+ | c1 | c2 | +------+------+ | v1 | v2 | +------+------+

Query attributes are subject to these limitations and restrictions:

To access query attributes within SQL statements for which attributes have been defined, thequery_attributescomponent must be installed. Do so using this statement:

INSTALL COMPONENT "file://component_query_attributes";

Component installation is a one-time operation that need not be done per server startup.INSTALL COMPONENTloads the component, and also registers it in themysql.componentsystem table to cause it to be loaded during subsequent server startups.

Thequery_attributescomponent accesses query attributes to implement amysql_query_attribute_string()function. SeeSection 5.5.4, “Query Attribute Components”.

To uninstall thequery_attributescomponent, use this statement:

UNINSTALL COMPONENT "file://component_query_attributes";

UNINSTALL COMPONENTunloads the component, and unregisters it from themysql.componentsystem table to cause it not to be loaded during subsequent server startups.

Because installing and uninstalling thequery_attributescomponent installs and uninstalls themysql_query_attribute_string()function that the component implements, it is not necessary to useCREATE FUNCTIONorDROP FUNCTIONto do so.