SELECT w.name AS Наименование, 
    w.material AS Материал,
    sum(sum) AS Сумма, 
    sum(reports.count) AS Количество 
    from reports
JOIN workpiece w ON reports.item_id = w.work_id
WHERE report_category_id = '$id' AND (month = '$month' AND year = '$year')
GROUP BY w.material, w.name;