`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;

(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');

(

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