RIS/App/Report/sql/sellers_report.sql

SELECT
    s.name AS Поставщик,
    SUM(reports.count) AS 'Количество поставок',
    SUM(reports.sum) AS Сумма
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;