Another Prospective for the Solution

I am quite confused regarding the solution as the given solution is just checking the year when an employee starteed working 

However, maybe there are some employees that are employed before 1990 but still working in the company by that year and even after

and this can be applied to all years

Also there is nothing mentioned regarding when the employee has left

I had a solution trying to consider those factors, it seems quite complicated and do not know if I am right or not

SELECT 
    cal_year,
    gender,
    SUM(CASE
        WHEN cal_year BETWEEN YEAR(hire_date) AND YEAR(end_date) THEN 1
        ELSE 0
    END) AS no_of_emp
FROM
    (SELECT 
        e.emp_no, e.gender, e.hire_date, MAX(d.to_date) AS end_date
    FROM
        employees e
    JOIN dept_emp d ON e.emp_no = d.emp_no
    GROUP BY e.emp_no) a
        CROSS JOIN
    (SELECT DISTINCT
        YEAR(hire_date) AS cal_year
    FROM
        employees
    WHERE
        YEAR(hire_date) >= 1990) b
GROUP BY cal_year , gender
ORDER BY cal_year;