MySQL预处理语句PREPARE、EXECUTE、DEALLOCATE使用大全

说明

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：

1. 准备好的语句在其他会话无效；

2. 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。

3. 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。

示例

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)

示例存储过程

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;