MySQL 5.7 PREPARE、EXECUTE、DEALLOCATE語句介紹

PREPARE語句準備好一條SQL語句，並分配給這條SQL語句一個名字供之後呼叫。準備好的SQL語句透過EXECUTE命令執行，透過DEALLOCATE PREPARE命令釋放掉。

語句的名字不區分大小寫。準備好的SQL語句名字可以是字串，也可以是使用者指定的包含SQL文字的變數。PREPARE中的SQL文字必須代表一條單獨的SQL語句而不能是多條SQL語句。在SQL語句中，? 字元用來作為後面執行查詢使用的一個引數。? 不能加上引號，及時打算將它們繫結到字元變數中也不可以。

如果準備好的SQL語句名字已經存在，它會在新語句被準備好前釋放掉。這意味著，如果一條新的語句包含錯誤且不能被準備好，這時會返回錯誤並且準備好的SQL語句將不再存在。

準備好的語句範圍是建立它的會話，具有下列特點：

準備好的語句在其他會話無效；

當會話結束時，不管會話時正常結束還是異常結束，這個會話中準備好的SQL語句將不再存在。如果自動連線功能開啟，客戶端不會被通知連線丟失。

在儲存過程或函式里面的準備好的語句，在儲存過程或函式執行結束後，會繼續存在，可以在儲存過程或包外面繼續被執行。

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)

示例儲存過程
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 ;