SELECT c.id, v.id
      FROM (countries c LEFT JOIN item_country_map i1 ON c.id=i1.country_id)
CROSS JOIN (variables v LEFT JOIN item_variable_map i2 ON v.id=i2.variable_id)
  GROUP BY c.id, v.id
    HAVING COUNT(IF(i1.item_id=i2.item_id,1,null))=0