如何基于数据库中的列排序日期，最新日期在最前面

我的数据库中有6个不同的表，所有6个表中都有一列“ created_at”。我想对我的回复进行排序，以使最新帖子排在最前面。我正在使用orderBy（'created_at'），但返回的顺序错误。我的数据库中具有“ timestamp”类型的“ created_at”列。它将按此顺序退回该职位2019年10月1日至6日，2019年10月2日至7日，2019年9月3日至23日，2019年9月4日至26日，2019年9月5日至27日等等。这是我的代码：-

public function wsUserActivity(){
    $request = Input::all();
    try {
        $user_id = $request['user_id'];
        $no=isset($request['page_number'])?$request['page_number']:0;
        $nos=isset($request['count'])?$request['count']:10;
        $skp=$no*$nos;
        $array_json_return = array('status' => '1','msg' => 'Success');


        $u_activity = array();
        $u_article = DB::table('mst_article as article')
        ->select(DB::raw('"article" as type'),'id','title', DB::raw('DATE_FORMAT(created_at, "%d %b %Y") as created_at'), DB::raw('DATE_FORMAT(updated_at, "%d %b %Y") as updated_at'), 'imported', 'import_url', 'cover_type', 'profile_image')
        ->selectRaw('SUBSTRING(`description`, 1, 200) as `description`')
        ->where('user_id_fk',$user_id)
        ->where('status', '=', '1');




        $u_meetup = DB::table('mst_meetup as meetup')
        ->select(DB::raw('"meetup" as type'),'id','title', DB::raw('DATE_FORMAT(created_at, "%d %b %Y") as created_at'), DB::raw('DATE_FORMAT(updated_at, "%d %b %Y") as updated_at'), 'imported', 'import_url', 'cover_type', 'profile_image' )
        ->selectRaw('SUBSTRING(`description`, 1, 200) as `description`')
        ->where('user_id_fk',$user_id)
        ->where('status', '=', '1');



        $u_question = array();
        $u_question = DB::table('mst_question as question')
        ->select(DB::raw('"question" as type'),'id','title', DB::raw('DATE_FORMAT(created_at, "%d %b %Y") as created_at'), DB::raw('DATE_FORMAT(updated_at, "%d %b %Y") as updated_at'), 'imported', 'import_url', 'cover_type', 'profile_image')
        ->selectRaw('SUBSTRING(`description`, 1, 200) as `description`')
        ->where('user_id_fk',$user_id)
        ->where('status', '=', '1');


        $u_job = array();
        $u_job = DB::table('mst_job as job')
        ->select(DB::raw('"job" as type'),'id','title', DB::raw('DATE_FORMAT(created_at, "%d %b %Y") as created_at'), DB::raw('DATE_FORMAT(updated_at, "%d %b %Y") as updated_at'), 'imported', 'import_url', 'cover_type', 'profile_image')
        ->selectRaw('SUBSTRING(`description`, 1, 200) as `description`')
        ->where('user_id_fk',$user_id)
        ->where('status', '=', '1');



        $u_education = array();
        $u_education = DB::table('mst_education as education')
        ->select(DB::raw('"education" as type'),'id','title', DB::raw('DATE_FORMAT(created_at, "%d %b %Y") as created_at'), DB::raw('DATE_FORMAT(updated_at, "%d %b %Y") as updated_at'), 'imported', 'import_url', 'cover_type', 'profile_image')
        ->selectRaw('SUBSTRING(`description`, 1, 200) as `description`')
        ->where('user_id_fk',$user_id)
        ->where('status', '=', '1');


        $u_activity= DB::table('mst_event as event')
        ->select(DB::raw('"event" as type'),'id','title', DB::raw('DATE_FORMAT(created_at, "%d %b %Y") as created_at'), DB::raw('DATE_FORMAT(updated_at, "%d %b %Y") as updated_at'), 'imported', 'import_url', 'cover_type', 'profile_image' )
        ->selectRaw('SUBSTRING(`description`, 1, 200) as `description`')
        ->where('user_id_fk',$user_id)
        ->where('status', '=', '1')
        ->union($u_article)->union($u_question)->union($u_meetup)->union($u_job)->union($u_education)
        ->orderBy('created_at')
        ->skip($skp)
        ->take($nos)
        ->get();

        if (count($u_activity)>0){
            foreach ($u_activity as $key => $value){
                if (!empty($value->profile_image)){
                $u_activity[$key]->profile_image_url = config("feature_pic_url").$value->type.'_image/thumb/'.$value->profile_image;
                }
                $u_activity[$key]->post_url = url('/') . '/view-'.$value->type.  '/' . $value->id;
            }
        }



        $array_json_return['u_activity'] = $u_activity;


    } catch (\Exception $e) {
        $array_json_return = $this->api_default_fail_response(__function__, $e);
    }

    echo json_encode($array_json_return);
}