need help with grouping sets in sql - oracle

[sia]

Hi,

I actually found out a way to do it couple of minutes back but haven't understood the concept behind using the grouping function in having clause.....and what makes it work in this query.....

Select extract(year from order_date) "Year", sales_rep_id "Salesperson", customer_id "Customer", sum(order_total) "Order Total", count(*) "Orders"
FROM orders
WHERE order_mode = 'direct'
GROUP BY grouping sets (customer_id, sales_rep_id, rollup(extract(year from order_Date)))
HAVING customer_id BETWEEN 100 AND 105
OR sales_rep_id BETWEEN 155 AND 160
or extract(year from sysdate) >= extract(year from order_date)
or grouping (customer_id) > 0
and grouping (sales_rep_id) > 0
/
      Year Salesperson   Customer Order Total     Orders
---------- ----------- ---------- ----------- ----------
                              101    190395.1          4
                              102     69211.4          4
                              103     20591.4          4
                              104       52092          3
                              105     31902.7          3
                   155               134415.2          5
                   156               202617.6          5
                   158               156296.2          7
                   159               151167.2          7
                   160                88238.4          6
      1990                            61655.7          3
      1996                             5546.6          1
      1997                                310          1
      1998                           309929.8         10
      1999                          1274078.8         49
      2000                           252108.3          9
                                    1903629.2         73
17 rows selected.

best regards