How to count all fields?

Question

MySQL

It is necessary to calculate how many masters for each city with one request, why the calculation is not combined and how to do it?

CREATE TABLE `master_user` (
  `id` int(10) UNSIGNED NOT NULL,
  `user_id` int(11) NOT NULL,
  `master_id` int(11) NOT NULL,
  `city_id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

--
-- Дамп данных таблицы `master_user`
--

INSERT INTO `master_user` (`id`, `user_id`, `master_id`, `city_id`) VALUES
(15, 16, 3, 4019),
(18, 16, 5, 4019),
(19, 16, 4, 4019),
(20, 16, 2, 4019),
(21, 17, 4, 4019),
(22, 17, 2, 4400);

--
-- Индексы сохранённых таблиц
--

--
-- Индексы таблицы `master_user`
--
ALTER TABLE `master_user`
  ADD PRIMARY KEY (`id`);

--
-- AUTO_INCREMENT для сохранённых таблиц
--

--
-- AUTO_INCREMENT для таблицы `master_user`
--
ALTER TABLE `master_user`
  MODIFY `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=23;
COMMIT;

SELECT master_id, COUNT(*) FROM master_user WHERE city_id = 4019 GROUP BY id

Answer 1

SELECT master_id, COUNT(*) FROM master_user WHERE city_id = 4019 GROUP BY master_id

Answer 2

If we count the number of masters in cities, then
Select city_id, count(master_id) from master_user group by city_id