SELECT
    s.name AS Поставщик,
    reports.sum AS 'Сумма в рублях',
    reports.count AS 'Количество поставок'
FROM reports
JOIN sellers s ON reports.item_id = s.sel_id
WHERE report_category_id = '$id' AND (month = '$month' AND year = '$year')
GROUP BY s.name
ORDER BY s.name;