SELECT
[Department Name],
[Category Name],
CAST(ROUND(CAST(Cnt AS DECIMAL(16,4))/CntMax*100,0) AS INT) AS Percentage
 FROM(
SELECT d.Name AS [Department Name],
c.Name AS [Category Name],
COUNT(*) AS Cnt,
(SELECT COUNT(*) FROM Reports  AS r1 JOIN Categories
AS c1 ON c1.Id = r1.CategoryId  WHERE c1.DepartmentId=c.DepartmentId) AS CntMax
FROM Reports AS r
JOIN Categories AS c
ON c.Id = r.CategoryId
JOIN Departments AS d
ON d.Id = c.DepartmentId
GROUP BY c.DepartmentId,d.Name, c.Name
) T
ORDER BY [Department Name],
[Category Name],
Percentage