'on子句'中的MySQL未知列:((MySQL Unknown column in 'on clause' :()

 我的查询看起来像 
 
SELECT а.*, m.username, m.picture, m.picture_active
FROM questions_answer AS а 
INNER JOIN members AS m ON а.poster_id=m.member_id
INNER JOIN questions AS q ON q.question_id=a.question_id
ORDER BY a.postdate DESC
 
 我收到错误： 
 
Unknown column 'a.question_id' in 'on clause'
 
 我不知道有什么问题，请帮帮我。 
 
 表questions是 
 
CREATE TABLE IF NOT EXISTS `questions` (
  `question_id` int(9) unsigned NOT NULL AUTO_INCREMENT,
  `member_id` int(9) unsigned NOT NULL DEFAULT '0',
  `question` text NOT NULL,
  `postdate` int(10) unsigned NOT NULL DEFAULT '0',
  `active` tinyint(1) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`question_id`),
  KEY `member_id` (`member_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
 
 和questions_answer是 
 
CREATE TABLE IF NOT EXISTS `questions_answer` (
  `answer_id` bigint(12) unsigned NOT NULL AUTO_INCREMENT,
  `question_id` int(9) unsigned NOT NULL,
  `poster_id` int(9) unsigned NOT NULL,
  `body` text NOT NULL,
  `postdate` int(9) unsigned NOT NULL,
  PRIMARY KEY (`answer_id`),
  KEY `question_id` (`question_id`),
  KEY `poster_id` (`poster_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

My query looks like
 
SELECT а.*, m.username, m.picture, m.picture_active
FROM questions_answer AS а 
INNER JOIN members AS m ON а.poster_id=m.member_id
INNER JOIN questions AS q ON q.question_id=a.question_id
ORDER BY a.postdate DESC
 
I'm getting error:
 
Unknown column 'a.question_id' in 'on clause'
 
I don't know what is wrong, please help me with this.
 
Table questions is
 
CREATE TABLE IF NOT EXISTS `questions` (
  `question_id` int(9) unsigned NOT NULL AUTO_INCREMENT,
  `member_id` int(9) unsigned NOT NULL DEFAULT '0',
  `question` text NOT NULL,
  `postdate` int(10) unsigned NOT NULL DEFAULT '0',
  `active` tinyint(1) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`question_id`),
  KEY `member_id` (`member_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
 
and questions_answer is
 
CREATE TABLE IF NOT EXISTS `questions_answer` (
  `answer_id` bigint(12) unsigned NOT NULL AUTO_INCREMENT,
  `question_id` int(9) unsigned NOT NULL,
  `poster_id` int(9) unsigned NOT NULL,
  `body` text NOT NULL,
  `postdate` int(9) unsigned NOT NULL,
  PRIMARY KEY (`answer_id`),
  KEY `question_id` (`question_id`),
  KEY `poster_id` (`poster_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

原文：https://stackoverflow.com/questions/18814468