oracle技术之group by rollup查询结构

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　　在047考题中有以下这么一道考题
　　11.View the Exhibit and examine the descriptions of ORDER_ITEMS and ORDERS tables.
　　You want to display the CUSTOMER_ID, PRODUCT_ID, and total (UNIT_PRICE multiplied by
　　QUANTITY) for the order placed. You also want to display the subtotals for a CUSTOMER_ID as well as
　　for a PRODUCT_ID for the last six months.
　　Which SQL statement would you execute to get the desired output?
　　A. SELECT o.customer_id, oi.product_id, SUM(oi.unit_price*oi.quantity) "Total"
　　FROM order_items oi JOIN orders o
　　ON oi.order_id=o.order_id GROUP BY ROLLUP (o.customer_id,oi.product_id) WHERE MONTHS_BETWEEN(order_date, SYSDATE)