16 problem from exam 22.10.2017 sql

SELECT d.Name,CASE WHEN AVG(aa.Difference) IS NULL THEN 'no info' ELSE CAST(AVG(aa.Difference)AS VARCHAR )
        END AS [Average Duration]
 FROM Departments AS d
JOIN (SELECT c.DepartmentId,DATEDIFF(DAY,r.OpenDate,r.CloseDate)AS 'Difference' FROM  Categories AS c
JOIN Reports AS r ON  r.CategoryId=c.Id
GROUP BY c.DepartmentId,r.OpenDate,r.CloseDate)AS aa ON d.Id=aa.DepartmentId
GROUP BY d.Name
ORDER BY d.Name