SELECT d.name,
IF( AVG(DATEDIFF(r.close_date, r.open_date)), FLOOR(AVG(DATEDIFF(r.close_date, r.open_date))), 'no info') AS `average_duration`
FROM departments AS d
JOIN categories AS c ON c.department_id = d.id
JOIN reports AS r ON r.category_id = c.id
GROUP BY d.name
ORDER BY d.name;