php递归函数查询mysql数据，获取所有上下级

 /*
  * 获取所有上级
  * @param $id String 待查找的id
  * @return String | NULL 失败返回null
  */
  public function getSup($id,$n = 0)
    {
        $sql = "SELECT `pid` from `aaa` where `id` =" . $id;
        $res = $this->db->getone($sql);   //获取字段值，如15
        if ($res) {
            if($n){
                $ids .= "," . $res;
            }else{
                $ids = $res;
            }
            $n++;
            $ids .= $this->test($res,$n);
        } 
        return $ids;
    }

   /*
    * 获取所有下级
    * @param $id String 待查找的id
    * @return String | NULL 失败返回null
    */
    public function getSub($id)
    {
        $isComma = strstr($id,',');
        if($isComma){
            $sql = "SELECT `id` from `aaa` where `pid` in (" . $id.")";
        }else{
            $sql = "SELECT `id` from `aaa` where `pid`=" . $id;
        }
        
        $res = $this->sql($sql);     //获取二维数组，如Array ( [0] => Array ( [id] => 12 ) [1] => Array ( [id] => 13 ) )
        
        if ($res) {
            $id = '';
            foreach ($res as $k=>$v) {
                if($v['id']>0){
                    if($k == 0){
                        $id =  $v['id'];
                    }else{
                        $id .=  ','.$v['id'];
                    }
                }
            }
            if($isComma){
                $ids .= "," . $id;
            }else{
                $ids .=  $id;
            }
            $ids .= $this->test2($id);
        }
        return $ids;
    }

数据库

-- ----------------------------
-- Table structure for aaa
-- ----------------------------
DROP TABLE IF EXISTS `aaa`;
CREATE TABLE `aaa` (
  `id` int(11) NOT NULL,
  `nodename` varchar(20) DEFAULT NULL,
  `pid` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

-- ----------------------------
-- Records of aaa
-- ----------------------------
INSERT INTO `aaa` VALUES ('1', 'A', '0');
INSERT INTO `aaa` VALUES ('2', 'B', '1');
INSERT INTO `aaa` VALUES ('3', 'C', '1');
INSERT INTO `aaa` VALUES ('4', 'D', '2');
INSERT INTO `aaa` VALUES ('5', 'E', '2');
INSERT INTO `aaa` VALUES ('6', 'F', '3');
INSERT INTO `aaa` VALUES ('7', 'G', '6');
INSERT INTO `aaa` VALUES ('8', 'H', '0');
INSERT INTO `aaa` VALUES ('9', 'I', '8');
INSERT INTO `aaa` VALUES ('10', 'J', '8');
INSERT INTO `aaa` VALUES ('11', 'K', '8');
INSERT INTO `aaa` VALUES ('12', 'L', '9');
INSERT INTO `aaa` VALUES ('13', 'M', '9');
INSERT INTO `aaa` VALUES ('14', 'N', '12');
INSERT INTO `aaa` VALUES ('15', 'O', '12');
INSERT INTO `aaa` VALUES ('16', 'P', '15');
INSERT INTO `aaa` VALUES ('17', 'Q', '0');

mysql with 查询上下级