根据一个分类id 获取这个分类底下所有子分类的商品信息，根据下面方法查询出所有有关分类id 再 根据这些id去商品表里查询所有商品信息

/**

 * 检测该分类下所有子分类，并输出ID（包括自己)
 * 数据库字段 catid pid
 */
function getChildrenIds ($sort_id){
	include_once APPPATH.‘/libraries/db.php‘;
	$db = new Db();
	$ids = $sort_id;
	$sql = "SELECT catid,pid FROM jy_category WHERE pid =".$sort_id;
	$result = $db->select($sql);

	if ($result){
		foreach ($result as $key=>$val){
			$ids .= ‘,‘.$val[‘catid‘].",";
			$ids .= getChildrenIds ($val[‘catid‘]);
		}
	}

	$idsArr = explode(",",$ids);
	$idsArr = array_unique($idsArr);
	$ids = implode(",",$idsArr);
	return $ids;
}

/**
 * 检测该分类下所有子分类，并输出ID（不包括自己)
 * 数据库字段 catid pid
 */
function getChildrenIds ($sort_id)
   {
       $db = $this->loadDB();
       $ids = ‘‘;
       $sql = "SELECT * FROM t_dept WHERE `parent_id` = ‘{$sort_id}‘";
       $query = $db->query($sql);
       $result = $query->result_array();

       if ($result)
       {
           foreach ($result as $key=>$val)
           {
               $ids .= ‘,‘.$val[‘id‘];
               $ids .= $this->getChildrenIds ($val[‘id‘]);
           }
       }
       return $ids;
}