如$id = 14
public function selectRosterList($id)
    {
        $data = db('department')->where('pid',$id)->select();
        $rosters = [];
        foreach ($data as $key=>$value){
            $roster = db('roster')->where('did',$value['id'])->select();
            $rosters = array_merge($rosters, $roster);
            if ($value['pid'] != 0){
                $this->selectRosterList($value['id']);
            }
        }
        return $rosters;
    }

为什么我这样写只能得到张三和李四的数据，却得不到小明的数据？