MySQL 8.0

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CREATE TABLE `employees` (
  `id` int(10) UNSIGNED NOT NULL,
  `name` varchar(50) NOT NULL,
  `start_date` date NOT NULL,
  `end_date` date NOT NULL,
  `salary` decimal(10,2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO `employees` (`id`, `name`, `start_date`, `end_date`, `salary`) VALUES
(1, 'Mark', '2017-05-01', '2020-01-31', '2000.00'),
(2, 'Tania', '2018-02-01', '2019-08-31', '5000.00'),
(3, 'Leo', '2018-02-01', '2018-09-30', '3000.00'),
(4, 'Elsa', '2018-12-01', '2020-05-31', '4000.00');
WITH RECURSIVE months (year, month) AS
(
  SELECT YEAR(MIN(start_date)) AS year, MONTH(MIN(start_date)) AS month FROM employees
  UNION ALL
  SELECT year + (month = 12), (month % 12) + 1 FROM months
  WHERE STR_TO_DATE(CONCAT_WS('-', year, month, '01'), '%Y-%m-%d') <= (SELECT MAX(end_date) FROM employees)
)
SELECT m.year, m.month, ROUND(AVG(e.salary), 2) AS avg_salary
FROM months m
LEFT JOIN employees e ON STR_TO_DATE(CONCAT_WS('-', m.year, m.month, '01'), '%Y-%m-%d') BETWEEN e.start_date AND e.end_date
WHERE STR_TO_DATE(CONCAT_WS('-', m.year, m.month, '01'), '%Y-%m-%d') BETWEEN '2018-08-01' AND '2019-01-31'
GROUP BY m.year, m.month


year
month
avg_salary


2018
8
3333.33

2018
9
3333.33

2018
10
3500.00

2018
11
3500.00

2018
12
3666.67

2019
1
3666.67

year	month	avg_salary
2018	8	3333.33
2018	9	3333.33
2018	10	3500.00
2018	11	3500.00
2018	12	3666.67
2019	1	3666.67