Consider the star schema from Figure 11.2. Suppose an analyst finds that monthly total sales (sum of the price values of all sales tuples) have decreased, instead of growing, from April 2018 to May 2018. The analyst wishes to check if there are specific item categories, stores, or customer countries that are responsible for the decrease.

a. What are the aggregates that the analyst would start with, and what are the relevant drill-down operations that the analyst would need to execute?

b. Write an SQL query that shows the item categories that were responsible for the decrease in sales, ordered by the impact of the category on the sales decrease, with categories that had the highest impact sorted first.

item_info item_id itemname color size category date_info date month quarter year sales item_id store_id customer_id date number price store store_id city state country customer customer_id name street city state zipcode country