#1

SELECT 
    AVG(DISTINCT salary)
FROM
    employees;

#2

SELECT 
    ROUND(AVG(DISTINCT salary), 2)
FROM
    employees;

#3

SELECT
	AVG(DISTINCT salary)
FROM
	employees
WHERE
	department_id = 5;

#4

SELECT 
    AVG(salary)
FROM
    employees
WHERE
    job_id = 6;

#5

SELECT
	department_id,
	AVG(salary)
FROM
	employees
GROUP BY
	department_id;

#6

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

#7

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

#8

SELECT
	AVG(employee_sal_avg)
FROM
	(
		SELECT
			AVG(salary) employee_sal_avg
		FROM
			employees
		GROUP BY
			department_id
	) t;