SELECT  d.name, c.name as category_name,
ROUND(
   (COUNT(r.id) / (SELECT COUNT(r.id) FROM reports as r
                   JOIN categories as c ON r.category_id = c.id
                   WHERE c.department_id = d.id)) * 100 /*get percentage here, then we round*/
, 0) AS percentage FROM reports as r
JOIN categories as c ON r.category_id = c.id
JOIN departments as d ON c.department_id = d.id
GROUP BY c.name
ORDER BY d.name, c.name, percentage;