我正在尝试在存储过程中执行prepared语句。
但我对此有意见。
以下是存储过程：

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `getAllUserLunchReport`(IN `start_date` DATETIME, IN `end_date` DATETIME)
BEGIN

SET GLOBAL group_concat_max_len=4294967295;
SET @SQL = NULL;
SET @start_date = DATE(start_date);
SET @end_date = DATE(end_date);

    SELECT
    GROUP_CONCAT(DISTINCT
        CONCAT(
        'SUM(CASE WHEN date = "',DATE(issuedDateTime),'" THEN lunchStatus ELSE 0 END) AS `',DATE(issuedDateTime),'`'
        )
    ) INTO @SQL

        FROM `lunch_status` 
        WHERE DATE(issuedDateTime) BETWEEN @start_date AND @end_date;

    SET @SQL 
    = CONCAT('SELECT userId, ', @SQL, ' 
    FROM
    (
        SELECT userId, lunchStatus, DATE(issuedDateTime) as date 
        FROM `lunch_status` 
        WHERE DATE(issuedDateTime) BETWEEN "',@start_date,'" AND "',@end_date,'" 
    ) as a
    GROUP BY userId;');

PREPARE stmt FROM @SQL;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

END$$
DELIMITER ;

下面是错误：

PDOStatement: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'NULL' at line 1

这是控制器：

public function weeklyLunchReport($date1=null, $date2=null)
{
    $user = AclHelper::getCurrentUser();
    $default_userID = intval($user['user_id']);
    $defDate = new DateTime();
    $default_date = $defDate->format('Y-m-d');

    $date1 = (isset($date1) && $date1 != '') ? $date1 : $default_date;
    $date2 = (isset($date2) && $date2 != '') ? $date2 : $default_date;

    $records = $this->db->exec("CALL getAllUserLunchReport ('$date1','$date2')");
    return $records;
}

另外：下面是sql变量的值：

SELECT userId, 
SUM(CASE WHEN date = "2018-06-03" THEN lunchStatus ELSE 0 END) AS `2018-06-03`,
SUM(CASE WHEN date = "2018-06-04" THEN lunchStatus ELSE 0 END) AS `2018-06-04`,
SUM(CASE WHEN date = "2018-06-06" THEN lunchStatus ELSE 0 END) AS `2018-06-06`,
SUM(CASE WHEN date = "2018-06-07" THEN lunchStatus ELSE 0 END) AS `2018-06-07`,
SUM(CASE WHEN date = "2018-06-08" THEN lunchStatus ELSE 0 END) AS `2018-06-08` 
   FROM
   (
       SELECT userId, lunchStatus, DATE(issuedDateTime) as date 
       FROM `lunch_status` 
       WHERE DATE(issuedDateTime) BETWEEN "2018-06-03" AND "2018-06-09" 
   ) as a
   GROUP BY userId;

单独运行此查询字符串将得到预期的结果集。
这个问题看起来很好，但我不知道哪里出了问题。
非常感谢您的帮助。提前谢谢。