修改别人的东西看到这个：
$tmp_m_price = $this->meal_orderDb->where('mer_id=' . $this->merid . ' AND paid="1" AND status != 3')->field('sum(if(total_price>0,total_price,price)) as tprice,sum(minus_price) as offprice')->find();

field里面的if是怎么回事