mysql - 平均计算代码点火器查询

Iam尝试查找费率的平均值，但在codeignier查询结果下方出现错误““字段列表”中的未知列“ prate””

 $this->db->select('AVG(prate)as avgprate');
        $this->db->from('stock','(select sum((pdetails.quantity*pdetails.rate)/pdetails.quantity) AS prate FROM `stock` INNER JOIN `pdetails` ON `pdetails`.`stock_id` = `stock`.`stock_id`)');

        $result = $this->db->get();
        $res = $result->row();

我该如何解决

最佳答案

$this->db->select('AVG(t.prate)as avgprate');
        $this->db->from('stock ,(select sum((pdetails.quantity*pdetails.rate)/pdetails.quantity) AS prate FROM `stock` INNER JOIN `pdetails` ON `pdetails`.`stock_id` = `stock`.`stock_id`)t');

        $result = $this->db->get();
        $res = $result->row();