使用Codeigniter活动记录重写MySQL查询

我不久前就开始使用CodeIgniter。有人能告诉我如何重写这个查询吗

SELECT question.timestamp AS question_time, reply.timestamp AS reply_time, 
(SELECT DATEDIFF(reply_time, question_time)) AS time_used_to_reply 
FROM question JOIN reply ON question.id = reply.question_id WHERE question.company_id=someid

使用activerecord

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此外，任何能快速理解activerecord的教程都将不胜感激。谢谢

经过多次研究，我终于写下了自己的活动记录脚本

$this->db->select(array('DATEDIFF(reply.timestamp, question.timestamp)'))
    ->from('question')
    ->join('reply', 'question.id = reply.question_id')
    ->where('question.company_id', $company_id);

---

谢谢

经过多次研究，我终于写下了自己的活动记录脚本

$this->db->select(array('DATEDIFF(reply.timestamp, question.timestamp)'))
    ->from('question')
    ->join('reply', 'question.id = reply.question_id')
    ->where('question.company_id', $company_id);

谢谢