Use CUBE and ROLLUP to calculate aggregates at many levels in one statement:
http://docs.oracle.com/cd/E11882_01/server.112/e25554/aggreg.htm#DWHSG8613
ROLLUP example
==============
SELECT channel_desc, calendar_month_desc, countries.country_iso_code,
TO_CHAR(SUM(amount_sold), '9,999,999,999') SALES$
FROM sales, customers, times, channels, countries
WHERE sales.time_id=times.time_id AND sales.cust_id=customers.cust_id
AND customers.country_id = countries.country_id
AND sales.channel_id= channels.channel_id
AND channels.channel_desc IN ('Direct Sales', 'Internet')
AND times.calendar_month_desc IN ('2000-09', '2000-10')
AND countries.country_iso_code IN ('GB', 'US')
GROUP BY channel_desc, ROLLUP(calendar_month_desc, countries.country_iso_code);
CHANNEL_DESC CALENDAR CO SALES$
-------------------- -------- -- --------------
Internet 2000-09 GB 16,569
Internet 2000-09 US 124,224
Internet 2000-09 140,793
Internet 2000-10 GB 14,539
Internet 2000-10 US 137,054
Internet 2000-10 151,593
Internet 292,387
Direct Sales 2000-09 GB 85,223
Direct Sales 2000-09 US 638,201
Direct Sales 2000-09 723,424
Direct Sales 2000-10 GB 91,925
Direct Sales 2000-10 US 682,297
Direct Sales 2000-10 774,222
Direct Sales 1,497,646
CUBE example
==============
SELECT channel_desc, calendar_month_desc, countries.country_iso_code,
TO_CHAR(SUM(amount_sold), '9,999,999,999') SALES$
FROM sales, customers, times, channels, countries
WHERE sales.time_id=times.time_id AND sales.cust_id=customers.cust_id AND
sales.channel_id= channels.channel_id
AND customers.country_id = countries.country_id
AND channels.channel_desc IN
('Direct Sales', 'Internet') AND times.calendar_month_desc IN
('2000-09', '2000-10') AND countries.country_iso_code IN ('GB', 'US')
GROUP BY CUBE(channel_desc, calendar_month_desc, countries.country_iso_code);
CHANNEL_DESC CALENDAR CO SALES$
-------------------- -------- -- --------------
1,790,032
GB 208,257
US 1,581,775
2000-09 864,217
2000-09 GB 101,792
2000-09 US 762,425
2000-10 925,815
2000-10 GB 106,465
2000-10 US 819,351
Internet 292,387
Internet GB 31,109
Internet US 261,278
Internet 2000-09 140,793
Internet 2000-09 GB 16,569
Internet 2000-09 US 124,224
Internet 2000-10 151,593
Internet 2000-10 GB 14,539
Internet 2000-10 US 137,054
Direct Sales 1,497,646
Direct Sales GB 177,148
Direct Sales US 1,320,497
Direct Sales 2000-09 723,424
Direct Sales 2000-09 GB 85,223
Direct Sales 2000-09 US 638,201
Direct Sales 2000-10 774,222
Direct Sales 2000-10 GB 91,925
Direct Sales 2000-10 US 682,297
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