Applications that use the MySQL C API define query attributes by calling the mysql_bind_param() function. 使用MySQL C API的应用程序通过调用MySQL_bind_param()函数来定义查询属性。See mysql_bind_param(). Other MySQL connectors may also provide query-attribute support. See the documentation for individual connectors.请参见mysql_bind_param()。其他MySQL连接器也可以提供查询属性支持。有关各个连接器，请参阅文档。

The mysql client has a query_attributes command that enables defining up to 32 pairs of attribute names and values. See Section 4.5.1.2, “mysql Client Commands”.mysql客户端有一个query_attributes命令，可以定义多达32对属性名称和值。请参阅第4.5.1.2节，“mysql客户端命令”。

Query attribute names are transmitted using the character set indicated by the character_set_client system variable.查询属性名称使用character_set_client系统变量所指示的字符集进行传输。

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. 要访问已定义属性的SQL语句中的查询属性，请按照使用查询属性的前提条件中的说明安装query_attributes组件。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. 该组件实现了一个mysql_query_attribute_string()可加载函数，该函数接受一个属性名称参数并以字符串形式返回属性值，如果该属性不存在，则返回NULL。See Query Attribute Loadable Functions.请参见查询属性可加载函数。

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.以下示例使用mysql客户端query_attributes命令定义属性名称/值对，并使用mysql_query_aattribute_string()函数按名称访问属性值。

This example defines two attributes named n1 and n2. 此示例定义了两个名为n1和n2的属性。The first SELECT shows how to retrieve those attributes, and also demonstrates that retrieving a nonexistent attribute (n3) returns NULL. 第一个SELECT展示了如何检索这些属性，还展示了检索不存在的属性（n3）会返回NULL。The second SELECT shows that attributes do not persist across statements.第二个SELECT显示属性不会在语句之间持久存在。

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. 如第二个SELECT语句所示，在给定语句之前定义的属性仅对该语句可用，并在语句执行后清除。若要在多个语句中使用一个属性值，请将其分配给一个变量。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():以下示例显示了如何做到这一点，并说明了属性值在后续语句中可以通过变量获得，但不能通过调用mysql_query_attribute_string()获得：

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:还可以通过将属性存储在表中来保存属性以备日后使用：

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:查询属性受以下限制：

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:若要访问已定义属性的SQL语句中的查询属性，必须安装query_attributes组件。使用以下语句执行此操作：

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.组件安装是一次性操作，不需要每次启动服务器就完成。INSTALL COMPONENT加载组件，并将其注册在mysql.component系统表中，以便在随后的服务器启动过程中加载。

The query_attributes component accesses query attributes to implement a mysql_query_attribute_string() function. query_attributes组件访问查询属性以实现一个mysql_query_aattribute_string()函数。See Section 5.5.4, “Query Attribute Components”.请参见第5.5.4节，“查询属性组件”。

To uninstall the query_attributes component, use this statement:要卸载query_attributes组件，请使用以下语句：

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.UNINSTALL COMPONENT将卸载该组件，并从mysql.component系统表中注销该组件，以便在随后的服务器启动过程中不加载该组件。

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.因为安装和卸载query_attributes组件会安装和卸载该组件实现的mysql_query_aattribute_string()函数，所以不需要使用CREATE FUNCTION或DROP FUNCTION。