Calculate daily sums in PostgreSQL - postgresql

Question

I am fairly new in postgres and what am trying to do is calculate sum values for each day for every month (i.e daily sum values). Based on scattering information I came up with something like this:
CREATE OR REPLACE FUNCTION sumvalues() RETURNS double precision AS
$BODY$
BEGIN
FOR i IN 0..31 LOOP
SELECT SUM("Energy")
FROM "public"."EnergyWh" e
WHERE e."DateTime" = day('01-01-2005 00:00:00'+ INTERVAL 'i' DAY);
END LOOP;
END
$BODY$
LANGUAGE plpgsql VOLATILE NOT LEAKPROOF;
ALTER FUNCTION public.sumvalues()
OWNER TO postgres;
The query returned successfully, so I thought I had made it. However when am trying to insert the values of the function to a table (which maybe wrong):
INSERT INTO "SumValues"
("EnergyDC")
(
SELECT sumvalues()
);
I get this:
ERROR: invalid input syntax for type interval: "01-01-2005 00:00:00"
LINE 3: WHERE e."DateTime" = day('01-01-2005 00:00:00'+ INTERVAL...
I tried to debug it myself but yet am not sure, which of the two I am doing wrong (or both) and why.
Here is an example of EnergyWh
(am using systemid and datetime as composite PK, but that should not matter)

Answer 1

see GROUP BY clause http://www.postgresql.org/docs/9.2/static/tutorial-agg.html
SELECT EXTRACT(day FROM e."DateTime"), EXTRACT(month FROM e."DateTime"),
EXTRACT(year FROM e."DateTime"), sum("Energy")
FROM "public"."EnergyWh" e
GROUP BY 1,2,3
but following query should to work too:
SELECT e."DateTime"::date, sum("Energy")
FROM "public"."EnergyWh" e
GROUP BY 1
I am using a short syntax for GROUP BY ~ GROUP BY 1 .. group by first column.

Answer 2

Here is simple Example that can help you:
Table :
create table demo (value double precision);
Function
CREATE OR REPLACE FUNCTION sumvalues() RETURNS void AS
$BODY$
DECLARE
inte text;
BEGIN
FOR i IN 0..31 LOOP
inte := 'INSERT INTO demo SELECT EXTRACT (DAY FROM TIMESTAMP ''01-01-2005 00:00:00''+ INTERVAL '''||i||' Days'')';
EXECUTE inte;
END LOOP;
END
$BODY$
LANGUAGE plpgsql VOLATILE NOT LEAKPROOF;
ALTER FUNCTION public.sumvalues()
OWNER TO postgres;
Function Call
SELECT sumvalues();
Output
SELECT * FROM demo;
Here if you want to use some variable value into SQL query than you must have to use some DYNAMIC QUERY for that.
Reference : Dynamic query in pgsql