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

The mysql client has a query_attributes command that enables defining up to 32 pairs of attribute names and values. See Section 6.5.1.2, “mysql Client Commands”.

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

To access query attributes within SQL statements for which attributes have been defined, install the query_attributes component as described in Prerequisites for Using Query Attributes. The component implements a mysql_query_attribute_string() loadable function that takes an attribute name argument and returns the attribute value as a string, or NULL if the attribute does not exist. See Query Attribute Loadable Functions.

The following examples use the mysql client query_attributes command to define attribute name/value pairs, and the mysql_query_attribute_string() function to access attribute values by name.

This example defines two attributes named n1 and n2. The first SELECT shows how to retrieve those attributes, and also demonstrates that retrieving a nonexistent attribute (n3) returns NULL. The second SELECT shows 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 second SELECT statement, 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 calling mysql_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, the query_attributes component 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 COMPONENT loads the component, and also registers it in the mysql.component system table to cause it to be loaded during subsequent server startups.

The query_attributes component accesses query attributes to implement a mysql_query_attribute_string() function. See Section 7.5.4, “Query Attribute Components”.

To uninstall the query_attributes component, use this statement:

UNINSTALL COMPONENT "file://component_query_attributes";

UNINSTALL COMPONENT unloads the component, and unregisters it from the mysql.component system table to cause it not to be loaded during subsequent server startups.

Because installing and uninstalling the query_attributes component installs and uninstalls the mysql_query_attribute_string() function that the component implements, it is not necessary to use CREATE FUNCTION or DROP FUNCTION to do so.