根据当前一周获得一年的最后一周

Question

我正在尝试根据平均货币转换率更新表格。我想要做的是本周选择前一周的平均转换率并应用转换。

UPDATE cost,
(
SELECT yearweek(c.currency_date) w,
c.currency _currency, AVG(c.rate) rate
FROM currency c
GROUP BY w,_currency) src 

SET
cost.cost = cost.cost/src.rate,
cost.currencyid = 'EUR'
WHERE YEARWEEK(cost.occured_at)= ??

 and cost.currencyid = _currency

但我不知道如何根据当前的一周获得上周的平均费率