How to get the count of current month Sunday's in psql? - postgresql

Question

How to get total number of Sunday's for given date in postgresql

Answer 1

You need EXTRACT:
SELECT
EXTRACT(DOW FROM DATE '2011-02-16') = 0; -- 0 is Sunday
This can result in true or false, it's a sunday or it's not. I have no idea what you mean by "total number" because that will always be 0 (the date is not a sunday) or 1 (the given data is a sunday).
Edit: Something like this?
SELECT
COUNT(*)
FROM
generate_series(timestamp '2011-01-01', '2011-03-01', '1 day') AS g(mydate)
WHERE
EXTRACT(DOW FROM mydate) = 0;

Answer 2

The total number of Sundays for a given date can only be either 0 or 1.
But if you want the number of Sundays within a given date range, then your best bet is a calendar table. To find how many Sundays are in February this year, I'd just
select count(*)
from calendar
where cal_date between '2011-02-01' and '2011-02-28' and
day_of_week = 'Sun';
or
select count(*)
from calendar
where year_of_date = 2011 and
month_of_year = 2 and
day_of_week = 'Sun';
Here's a basic calendar table that you can start with. I also included a PostgreSQL function to populate the calendar table. I haven't tested this in 8.3, but I'm pretty sure I'm not using any features that 8.3 doesn't support.
Note that the "dow" parts assume your days are in English. But you can easily edit those parts to match any language. (I think. But I could be wrong about "easily".)
-- Table: calendar
-- DROP TABLE calendar;
CREATE TABLE calendar
(
cal_date date NOT NULL,
year_of_date integer NOT NULL,
month_of_year integer NOT NULL,
day_of_month integer NOT NULL,
day_of_week character(3) NOT NULL,
CONSTRAINT calendar_pkey PRIMARY KEY (cal_date),
CONSTRAINT calendar_check CHECK (year_of_date::double precision = date_part('year'::text, cal_date)),
CONSTRAINT calendar_check1 CHECK (month_of_year::double precision = date_part('month'::text, cal_date)),
CONSTRAINT calendar_check2 CHECK (day_of_month::double precision = date_part('day'::text, cal_date)),
CONSTRAINT calendar_check3 CHECK (day_of_week::text =
CASE
WHEN date_part('dow'::text, cal_date) = 0::double precision THEN 'Sun'::text
WHEN date_part('dow'::text, cal_date) = 1::double precision THEN 'Mon'::text
WHEN date_part('dow'::text, cal_date) = 2::double precision THEN 'Tue'::text
WHEN date_part('dow'::text, cal_date) = 3::double precision THEN 'Wed'::text
WHEN date_part('dow'::text, cal_date) = 4::double precision THEN 'Thu'::text
WHEN date_part('dow'::text, cal_date) = 5::double precision THEN 'Fri'::text
WHEN date_part('dow'::text, cal_date) = 6::double precision THEN 'Sat'::text
ELSE NULL::text
END)
)
WITH (
OIDS=FALSE
);
ALTER TABLE calendar OWNER TO postgres;
-- Index: calendar_day_of_month
-- DROP INDEX calendar_day_of_month;
CREATE INDEX calendar_day_of_month
ON calendar
USING btree
(day_of_month);
-- Index: calendar_day_of_week
-- DROP INDEX calendar_day_of_week;
CREATE INDEX calendar_day_of_week
ON calendar
USING btree
(day_of_week);
-- Index: calendar_month_of_year
-- DROP INDEX calendar_month_of_year;
CREATE INDEX calendar_month_of_year
ON calendar
USING btree
(month_of_year);
-- Index: calendar_year_of_date
-- DROP INDEX calendar_year_of_date;
CREATE INDEX calendar_year_of_date
ON calendar
USING btree
(year_of_date);
And a rudimentary function to populate the table. I haven't tested this in 8.3 either.
-- Function: insert_range_into_calendar(date, date)
-- DROP FUNCTION insert_range_into_calendar(date, date);
CREATE OR REPLACE FUNCTION insert_range_into_calendar(from_date date, to_date date)
RETURNS void AS
$BODY$
DECLARE
this_date date := from_date;
BEGIN
while (this_date <= to_date) LOOP
INSERT INTO calendar (cal_date, year_of_date, month_of_year, day_of_month, day_of_week)
VALUES (this_date, extract(year from this_date), extract(month from this_date), extract(day from this_date),
case when extract(dow from this_date) = 0 then 'Sun'
when extract(dow from this_date) = 1 then 'Mon'
when extract(dow from this_date) = 2 then 'Tue'
when extract(dow from this_date) = 3 then 'Wed'
when extract(dow from this_date) = 4 then 'Thu'
when extract(dow from this_date) = 5 then 'Fri'
when extract(dow from this_date) = 6 then 'Sat'
end);
this_date = this_date + interval '1 day';
end loop;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;