MySQL官方将PREPARE、EXECUTE、DEALLOCATE统称为PREPARE STATEMENT,我习惯称其为【预处理语句】。 其语法为:
PREPARE stmt_name FROM preparable_stmt EXECUTE stmt_name [USING @var_name [, @var_name] ...] {DEALLOCATE | DROP} PREPARE stmt_name
PREPARE语句准备好一条SQL语句,并分配给这条SQL语句一个名字供之后调用。准备好的SQL语句通过EXECUTE命令执行,通过DEALLOCATE PREPARE命令释放掉。
语句的名字不区分大小写。准备好的SQL语句名字可以是字符串,也可以是用户指定的包含SQL文本的变量。PREPARE中的SQL文本必须代表一条单独的SQL语句而不能是多条SQL语句。在SQL语句中,? 字符用来作为后面执行查询使用的一个参数。? 不能加上引号,即使打算将它们绑定到字符变量中也不可以。
如果准备好的SQL语句名字已经存在,它会在新语句被准备好前释放掉。这意味着,如果一条新的语句包含错误且不能被准备好,这时会返回错误并且准备好的SQL语句将不再存在。
准备好的语句范围是创建它的会话,具有下列特点:
准备好的语句在其他会话无效;
当会话结束时,不管会话是正常结束还是异常结束,这个会话中准备好的SQL语句将不再存在。如果自动连接功能开启,客户端不会被通知连接丢失。
在存储过程或函数里面的准备好的语句,在存储过程或函数执行结束后,会继续存在,可以在存储过程或包外面继续被执行。
mysql> SET @a=10; Query OK, 0 rows affected (0.00 sec) mysql> PREPARE STMT FROM 'SELECT * FROM dept2 LIMIT ?'; Query OK, 0 rows affected (0.08 sec) Statement prepared mysql> EXECUTE STMT USING @a; +--------+-------+ | deptno | dname | +--------+-------+ | 10 | A | | 20 | B | | 30 | C | | 40 | D | | 50 | E | | 60 | F | | 70 | G | | 80 | H | | 90 | I | | 100 | J | +--------+-------+ 10 rows in set (0.02 sec) mysql> SET @skip=1; SET @numrows=5; Query OK, 0 rows affected (0.00 sec) Query OK, 0 rows affected (0.00 sec) mysql> PREPARE STMT FROM 'SELECT * FROM dept2 LIMIT ?, ?'; Query OK, 0 rows affected (0.00 sec) Statement prepared mysql> EXECUTE STMT USING @skip, @numrows; +--------+-------+ | deptno | dname | +--------+-------+ | 20 | B | | 30 | C | | 40 | D | | 50 | E | | 60 | F | +--------+-------+ 5 rows in set (0.00 sec) mysql> DEALLOCATE PREPARE STMT; Query OK, 0 rows affected (0.00 sec) mysql> EXECUTE STMT USING @skip, @numrows; ERROR 1243 (HY000): Unknown prepared statement handler (STMT) given to EXECUTE mysql> PREPARE STMT FROM 'SELECT * FROM dept2 LIMIT ?, ?'; Query OK, 0 rows affected (0.00 sec) Statement prepared mysql> EXECUTE STMT USING @skip, @numrows; +--------+-------+ | deptno | dname | +--------+-------+ | 20 | B | | 30 | C | | 40 | D | | 50 | E | | 60 | F | +--------+-------+ 5 rows in set (0.00 sec)
delimiter $$ create procedure registerSQLReference( IN the_table_name VARCHAR(80), IN the_sql_script_version VARCHAR(80), IN the_sql_script_name VARCHAR(80), IN the_install_version VARCHAR(80), IN the_execution_duration VARCHAR(80), IN the_sql_script_description VARCHAR(200) ) BEGIN declare column_exist INT DEFAULT 0; declare the_query VARCHAR(500); select '[INFO] Check if the column C_CHANGE_DESCRIPTION exists in the table @VERSION_LEVEL_TABLE_NAME'; set column_exist = is_ChangeDescColumnExist ('@VERSION_LEVEL_TABLE_NAME'); IF column_exist = 0 THEN select '[INFO] VERSION LEVEL TABLE does not contain C_CHANGE_DESCRIPTION column.'; set @v_the_table_name=the_table_name; select concat('INSERT INTO ', @v_the_table_name, ' VALUES (?, ?, ?, ?, ?) ') into the_query; SET @stmt=the_query; PREPARE STMT FROM @stmt; select concat('[INFO] the_query=', the_query ); set @v_the_sql_script_version=the_sql_script_version; set @v_the_sql_script_name=the_sql_script_name; set @v_date=now(); set @v_the_install_version=the_install_version; set @v_the_execution_duration=the_execution_duration; EXECUTE STMT using @v_the_sql_script_version , @v_the_sql_script_name, @v_date, @v_the_install_version, @v_the_execution_duration; ELSE select '[INFO] VERSION LEVEL TABLE contains C_CHANGE_DESCRIPTION column.'; select concat('INSERT INTO ', the_table_name, ' VALUES (?, ?, ?, ?, ?, ?) ') into the_query; SET @stmt=the_query; PREPARE STMT FROM @stmt; select concat('[INFO] the_query=', the_query ); set @v_the_sql_script_version=the_sql_script_version; set @v_the_sql_script_name=the_sql_script_name; set @v_date=now(); set @v_the_install_version=the_install_version; set @v_the_execution_duration=the_execution_duration; set @v_the_sql_script_description=the_sql_script_description; select concat('[INFO] the_query=', the_query); EXECUTE the_query using @v_the_sql_script_version , @v_the_sql_script_name, @v_date, @v_the_install_version, @v_the_execution_duration, @v_the_sql_script_description; END IF; DEALLOCATE PREPARE STMT; END$$ delimiter;