Weekly total sums - sql

Question

I have a table in a PostgreSQL database containing dates and a total count per day.
mydate total
2012-05-12 12
2012-05-14 8
2012-05-13 4
2012-05-12 12
2012-05-15 2
2012-05-17 1
2012-05-18 1
2012-05-21 1
2012-05-25 1
Now I need to get the weekly totals for a given date range.
Ex. I want to get the weekly totals from 2012-05-01 up to 2012-05-31.
I'm looking at this output:
2012-05-01 2012-05-07 0
2012-05-08 2012-05-14 36
2012-05-15 2012-05-22 5
2012-05-23 2012-05-29 1
2012-05-30 2012-05-31 0

Answer 1

This works for any given date range:
CREATE FUNCTION f_tbl_weekly_sumtotals(_range_start date, _range_end date)
RETURNS TABLE (week_start date, week_end date, sum_total bigint)
LANGUAGE sql AS
$func$
SELECT w.week_start, w.week_end, COALESCE(sum(t.total), 0)
FROM (
SELECT week_start::date, LEAST(week_start::date + 6, _range_end) AS week_end
FROM generate_series(_range_start::timestamp
, _range_end::timestamp
, interval '1 week') week_start
) w
LEFT JOIN tbl t ON t.mydate BETWEEN w.week_start and w.week_end
GROUP BY w.week_start, w.week_end
ORDER BY w.week_start
$func$;
Call:
SELECT * FROM f_tbl_weekly_sumtotals('2012-05-01', '2012-05-31');
Major points
I wrapped it in a function for convenience, so the date range has to be provided once only.
The subquery w produces the series of weeks starting from the first day of the given date range. The upper bound is capped with LEAST to stay within the upper bound of the given date range.
Then LEFT JOIN to the data table (tbl in my example) to keep all weeks in the result, even where no data rows are found.
The rest should be obvious. COALESCE to output 0 instead of NULL for empty weeks.
Data types have to match, I assumed mydate date and total int for lack of information. (The sum() of an int is bigint.)
Explanation for my particular use of generate_series():
Generating time series between two dates in PostgreSQL

Answer 2

Using this function
CREATE OR REPLACE FUNCTION last_day(date)
RETURNS date AS
$$
SELECT (date_trunc('MONTH', $1) + INTERVAL '1 MONTH - 1 day')::date;
$$ LANGUAGE 'sql' IMMUTABLE STRICT;
AND generate_series (from 8.4 onwards) we can create the date partitions.
SELECT wk.wk_start,
CAST(
CASE (extract(month from wk.wk_start) = extract(month from wk.wk_start + interval '6 days'))
WHEN true THEN wk.wk_start + interval '6 days'
ELSE last_day(wk.wk_start)
END
AS date) AS wk_end
FROM
(SELECT CAST(generate_series('2012-05-01'::date,'2012-05-31'::date,interval '1 week') AS date) AS wk_start) AS wk;
Then putting it together with the data
CREATE TABLE my_tab(mydate date,total integer);
INSERT INTO my_tab
values
('2012-05-12'::date,12),
('2012-05-14'::date,8),
('2012-05-13'::date,4),
('2012-05-12'::date,12),
('2012-05-15'::date,2),
('2012-05-17'::date,1),
('2012-05-18'::date,1),
('2012-05-21'::date,1),
('2012-05-25'::date,1);
WITH month_by_week AS
(SELECT wk.wk_start,
CAST(
CASE (extract(month from wk.wk_start) = extract(month from wk.wk_start + interval '6 days'))
WHEN true THEN wk.wk_start + interval '6 days'
ELSE last_day(wk.wk_start)
END
AS date) AS wk_end
FROM
(SELECT CAST(generate_series('2012-05-01'::date,'2012-05-31'::date,interval '1 week') AS date) AS wk_start) AS wk
)
SELECT month_by_week.wk_start,
month_by_week.wk_end,
SUM(COALESCE(mt.total,0))
FROM month_by_week
LEFT JOIN my_tab mt ON mt.mydate BETWEEN month_by_week.wk_start AND month_by_week.wk_end
GROUP BY month_by_week.wk_start,
month_by_week.wk_end
ORDER BY month_by_week.wk_start;