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。请参见mysql_bind_param()。其他 MySQL 连接器也可能提供查询属性支持。请参阅各个连接器的文档。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.

mysql 客户端具有一个 query_attributes 命令，可定义多达 32 对属性名和属性值。请参见 第 6.5.1.2 节，“mysql 客户端命令”。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”.

查询属性名称是使用由 character_set_client 系统变量指示的字符集传输的。Query attribute names are transmitted using the character set indicated by the character_set_client system variable.

要在已定义属性的 SQL 语句中访问查询属性，请按照 使用查询属性的前提条件 中的说明安装 query_attributes 组件。该组件实现了一个 mysql_query_attribute_string() 可加载函数，该函数接受属性名称参数并返回属性值的字符串，如果属性不存在，则返回 NULL。请参见 查询属性可加载函数。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.

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

此示例定义了两个名为n1和n2的属性。第一个SELECT显示了如何检索这些属性，并演示了检索不存在的属性（n3）会返回NULL。第二个SELECT显示属性不会在语句之间持续存在。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   |
+--------+--------+

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，并且在语句执行后会被清除。要在多个语句中使用属性值，请将其赋值给变量。下面的示例演示了如何执行此操作，并说明属性值可以通过变量在后续语句中使用，但不能通过调用mysql_query_attribute_string()来获取：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   |
+--------+--------+--------+--------+

属性也可以通过将它们存储在表中以供以后使用：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   |
+------+------+

查询属性受以下限制和约束：Query attributes are subject to these limitations and restrictions:

要在已定义属性的 SQL 语句中访问查询属性，必须安装 query_attributes 组件。可使用以下语句进行安装：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";

组件安装是一次性操作，无需在每次服务器启动时执行。INSTALL COMPONENT 会加载组件，并将其注册到 mysql.component 系统表中，以便在后续服务器启动时加载该组件。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.

query_attributes 组件访问查询属性以实现 mysql_query_attribute_string() 函数。请参见 第 7.5.4 节，“查询属性组件”。The query_attributes component accesses query attributes to implement a mysql_query_attribute_string() function. See Section 7.5.4, “Query Attribute Components”.

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

UNINSTALL COMPONENT "file://component_query_attributes";

UNINSTALL COMPONENT 会卸载组件，并将其从 mysql.component 系统表中注销，以便在后续服务器启动时不加载该组件。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.

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_attribute_string() 函数，因此无需使用 CREATE FUNCTION 或 DROP FUNCTION 来执行此操作。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.