SELECT * FROM `cpl_mfo` JOIN `cpl_mfo_loans` ON `cpl_mfo_loans`.`mfo_id` = `cpl_mfo`.`id` WHERE MIN(`l_min_sum`) <= '999999' AND MAX(`l_max_sum`) >= '0' AND MIN(`l_min_time`) <= '999' AND MAX(`l_max_time`) >= '0' AND MIN(`l_min_age`) <= '100' AND MAX(`l_max_age`) >= '0' AND MIN(`l_min_percent`) <= '100' AND MAX(`l_max_percent`) >= '0' AND `how_get` LIKE '%%' AND `payments` LIKE '%%' AND `country` = 'Россия' GROUP BY `mfo_id` ORDER BY `rating` DESC
Group by здесь не причем. Убирал его, ошибка осталась.