join 单行数据 多行数据

    $this->db->order_by($key,$value);
}
//$this->db->order_by('id','desc');
if(!empty($category_id_arr)){
    $this->db->where_in('category_id', $category_id_arr);
}else{
}
$this->db->select('product.*,product_pingpai.pingpai_id');
$this->db->from('product');
$this->db->join('product_pingpai', 'product_pingpai.product_id = product.id', 'left');
$this->db->limit($limit, ($p-1)*$limit);
$query = $this->db->get();
if(!empty($category_id_arr)){
    $this->db->where_in('category_id', $category_id_arr);
}else{
}
return  $query->result_array(); //这是多行

//return $query->row_array(); //这是单行