Detta borde hjälpa dig
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