CREATE TABLE `table_tmp` SELECT DISTINCT * FROM `table`;
DROP TABLE `table`;
RENAME TABLE `table_tmp` TO `table`;
нет, с подзапросом не то…Да не вопрос:
SELECT o.*
FROM oper AS o
JOIN oper AS o2
ON o.id_country = o2.id_country
GROUP BY o.id_country, o.id_num
HAVING o.cost = MAX(o2.cost)