#1

SELECT 
    department_id
FROM 
    employees
GROUP BY 
    department_id;

#2

SELECT 
   DISTINCT department_id
FROM 
   employees
ORDER BY 
   department_id;

#3

SELECT
	department_id,
	COUNT(employee_id) headcount
FROM
	employees
GROUP BY
	department_id;

#4

SELECT
	department_name,
	COUNT(employee_id) headcount
FROM
	employees e
INNER JOIN departments d ON d.department_id = e.department_id
GROUP BY
        department_name;

#5

SELECT 
    department_name,
    COUNT(employee_id) headcount
FROM
    employees e
        INNER JOIN
    departments d ON d.department_id = e.department_id
GROUP BY department_name
ORDER BY headcount DESC;

#6

SELECT 
    department_name,
    COUNT(employee_id) headcount
FROM
    employees e
        INNER JOIN
    departments d ON d.department_id = e.department_id
GROUP BY department_name
HAVING headcount > 5
ORDER BY headcount DESC;

#7

SELECT 
    department_name,
    MIN(salary) min_salary,
    MAX(salary) max_salary,
    ROUND(AVG(salary), 2) average_salary
FROM
    employees e
        INNER JOIN
    departments d ON d.department_id = e.department_id
GROUP BY 
    department_name;

#8

SELECT 
    department_name,
    SUM(salary) total_salary
FROM
    employees e
        INNER JOIN
    departments d ON d.department_id = e.department_id
GROUP BY 
    department_name;

#9

SELECT 
    department_name,
    job_title,
    COUNT(employee_id)
FROM
    employees e
        INNER JOIN
    departments d ON d.department_id = e.department_id
        INNER JOIN
    jobs j ON j.job_id = e.job_id
GROUP BY department_name , 
         job_title;