--q75.sql--

 WITH all_sales AS (
    SELECT
        d_year, i_brand_id, i_class_id, i_category_id, i_manufact_id,
        SUM(sales_cnt) AS sales_cnt, SUM(sales_amt) AS sales_amt
    FROM (
        SELECT
            d_year, i_brand_id, i_class_id, i_category_id, i_manufact_id,
            cs_quantity - COALESCE(cr_return_quantity,0) AS sales_cnt,
            cs_ext_sales_price - COALESCE(cr_return_amount,0.0) AS sales_amt
        FROM catalog_sales
        JOIN item ON i_item_sk=cs_item_sk
        JOIN date_dim ON d_date_sk=cs_sold_date_sk
        LEFT JOIN catalog_returns ON (cs_order_number=cr_order_number
                                      AND cs_item_sk=cr_item_sk)
        WHERE i_category='Books'
        UNION
        SELECT
            d_year, i_brand_id, i_class_id, i_category_id, i_manufact_id,
             ss_quantity - COALESCE(sr_return_quantity,0) AS sales_cnt,
             ss_ext_sales_price - COALESCE(sr_return_amt,0.0) AS sales_amt
        FROM store_sales
        JOIN item ON i_item_sk=ss_item_sk
        JOIN date_dim ON d_date_sk=ss_sold_date_sk
        LEFT JOIN store_returns ON (ss_ticket_number=sr_ticket_number
                                    AND ss_item_sk=sr_item_sk)
        WHERE i_category='Books'
        UNION
        SELECT
            d_year, i_brand_id, i_class_id, i_category_id, i_manufact_id,
            ws_quantity - COALESCE(wr_return_quantity,0) AS sales_cnt,
            ws_ext_sales_price - COALESCE(wr_return_amt,0.0) AS sales_amt
        FROM web_sales
        JOIN item ON i_item_sk=ws_item_sk
        JOIN date_dim ON d_date_sk=ws_sold_date_sk
        LEFT JOIN web_returns ON (ws_order_number=wr_order_number
                                  AND ws_item_sk=wr_item_sk)
        WHERE i_category='Books') sales_detail
    GROUP BY d_year, i_brand_id, i_class_id, i_category_id, i_manufact_id)
 SELECT
    prev_yr.d_year AS prev_year, curr_yr.d_year AS year, curr_yr.i_brand_id,
    curr_yr.i_class_id, curr_yr.i_category_id, curr_yr.i_manufact_id,
    prev_yr.sales_cnt AS prev_yr_cnt, curr_yr.sales_cnt AS curr_yr_cnt,
    curr_yr.sales_cnt-prev_yr.sales_cnt AS sales_cnt_diff,
    curr_yr.sales_amt-prev_yr.sales_amt AS sales_amt_diff
 FROM all_sales curr_yr, all_sales prev_yr
 WHERE curr_yr.i_brand_id=prev_yr.i_brand_id
   AND curr_yr.i_class_id=prev_yr.i_class_id
   AND curr_yr.i_category_id=prev_yr.i_category_id
   AND curr_yr.i_manufact_id=prev_yr.i_manufact_id
   AND curr_yr.d_year=2002
   AND prev_yr.d_year=2002-1
   AND CAST(curr_yr.sales_cnt AS DECIMAL(17,2))/CAST(prev_yr.sales_cnt AS DECIMAL(17,2))<0.9
 ORDER BY sales_cnt_diff
 LIMIT 100