MYSQLを考慮に入れて最初の売上を考慮して顧客ごとの売上平均を計算することを選択します

これはあなたを助けるはずです

SELECT Customer, (total_sales/weeks) AS avg_sales FROM
(
SELECT Customer, total_sales, Date_First_Sale, IF(weeks>52,52,weeks) as weeks
FROM (
    SELECT Customer, SUM(Sales) AS total_sales, Date_First_Sale, TIMESTAMPDIFF(WEEK, Date_First_Sale, CURDATE()) AS weeks
    FROM (
        SELECT Customer, sales , Date_First_Sale
        FROM test.SO_customer
        WHERE Date > DATE_SUB(curdate(), INTERVAL 1 YEAR)
         ) as subTable
    GROUP BY Customer
    ) as subTable2
) as subTable3