我会交叉连接两个查询。第一个是每月销售产品数量的总和，第二个是当年销售产品总数的总和：

SELECT     `month`, `num_month` / `num_year` * 100 AS "percent"
FROM       (SELECT   MONTH(`dateOfOrder`) AS "month", COUNT(*) AS num_month
            FROM     `order`
            WHERE    YEAR(`dateOfOrder`) = 2018
            GROUP BY MONTH(`dateOfOrder`) a
CROSS JOIN (SELECT COUNT(*) AS num_year
            FROM   `order`
            WHERE  YEAR(`dateOfOrder`) = 2018) b