以下两种写法一致 ，后一种可以把users设置别名u

            $earliestTime = (new User())->leftJoin('ra_users_relation as ur', 'users.id', '=', 'ur.uid')
                ->where('ur.ra_uid', '=', Auth::user()['id'])
                ->min('users.created_at');

            $earliestTime = (new User())->from('users as u')->leftJoin('ra_users_relation as ur', 'u.id', '=', 'ur.uid')
                ->where('ur.ra_uid', '=', Auth::user()['id'])
                ->min('u.created_at');