Thread: Returning a set of dates

Hey folks,
  I am still slogging away on my pet project, and I'm giving up - I
need help from the experts to try to get this function working like
I want it to.
  I have a very basic function, thanks to you guys a few months ago:


CREATE OR REPLACE FUNCTION public.next_bill_date(d date, period interval,  i interval)RETURNS dateLANGUAGE plpgsql
AS $function$ DECLARE ret DATE; BEGIN
-- We must use "CURRENT_DATE + period" in SELECT below - if we just
-- use "d + i", we will only get the next billing date after the
-- bill's *epoch*.  Since we're passing an epoch, we need to
-- make sure we're getting the real next billing date
SELECT INTO ret generate_series( d, CURRENT_DATE + period, i ) AS
next_bill_date
ORDER BY next_bill_date DESC; RETURN ret;
END;
$function$

  If I call the function like so:


SELECT next_bill_date( '2011-06-10', '1 month', '1 year' );next_bill_date
----------------2011-06-10

  .. it gives me the *next date* a bill will be due.  Yay.  However,
several of my bills are recurring in a time period shorter than the
interval I will pass the function.  Ie, I have several savings "bills"
that occur twice a month, aligned with my paycheck.  If I call the
function:


SELECT next_bill_date( '2011-06-01', '2 weeks', '1 month' )

  .. I need it to return the two dates during the interval (1 month)
that this "bill" will be due.  I am brain-weary looking at this, so
in case my explanation is not clear, here's what I'm shooting for:

1) The function must accept an epoch date, when the bill "starts".  Ie,  my mortgage's epoch is on 2011-01-01, as it's
dueon the 1st of  the month.  The month and year aren't as critical, they just need  to represent a "starting date" in
thepast.  This epoch could just  as well be '2011-06-01'.  I manually enter the epochs, so I can make  the assumption
thatit will always be in the past.
 

2) The function must accept a period, or how often the bill recurs.  So, most bills will have a period of '1 month'.
Somemight be  '2 weeks'.  Some, like insurance, might be '6 months' or even  '1 year'.
 

3) The function must accept an interval, describing how long of a  time period we want to look at.  Ie, "I want to look
atall bills  over the next six months."  The interval would be '6 months'.  Or the upcoming bills over '6 weeks'.  You
getthe idea.
 

So, for example, if I call the function to determine my mortgage's
due dates over the next four months:


SELECT next_bill_date( '2011-01-01', '1 month', '4 months' );


.. I expect the following result set:

next_bill_date
----------------

2011-07-01
2011-08-01
2011-09-01
2011-10-01


I know I must use SETOF to return the set.  But I just can't seem to
get the damned syntax correct, and I'm more than a little lost trying
to get this function put together.  Can someone please help me out?

Thanks much!

Benny


-- 
"You were doing well until everyone died."                                   -- "God", Futurama

> http://www.postgresql.org/docs/8.4/interactive/plpgsql-control-structures.html#PLPGSQL-STATEMENTS-RETURNING
>
>CREATE OR REPLACE FUNCTION public.next_bill_date(d date, period
>interval,
>   i interval)
>  RETURNS SETOF date
> AS $function$
> DECLARE
>     max_date date;
>     due_date date;
> BEGIN
>     max_date := CURRENT_DATE + i;
>     due_date := d;
>     WHILE due_date + period <= max_date LOOP
>         RETURN NEXT due_date; -- add d to the result set
>         due_date := due_date + period;
>     END LOOP;
>     RETURN; -- exit function
> END;
> $function$ language plpgsql;
> testdb=# select next_bill_date('2011-06-11', '2 week', '3 month');
next_bill_date
> ----------------
>  2011-06-11
>  2011-06-25
>  2011-07-09
>  2011-07-23
>  2011-08-06
>  2011-08-20

Almost, but not quite - the d parameter is a bill's "start date",
and the function shouldn't show dates in the past.  So, when the
above function is called with say '2011-06-01' as the beginning
date, the function will happily return '2011-06-01' in the result
set, even though it's in the past.

I've modified it a bit.  I renamed the function arguments to be a
bit more descriptive, did a little more math, and added an IF
statement to not return any dates in the past:


CREATE OR REPLACE FUNCTION public.next_bill_date(d date, frequency  interval, daterange interval)RETURNS SETOF date
AS $function$
DECLARE   max_date date;   due_date date;
BEGIN   -- We need to add the epoch date and daterange together, to   -- get the "max_date" value.  However, this would
causeus   -- to lose the last due date in the result set.  Add one more   -- frequency to it so we don't lose that.
max_date:= CURRENT_DATE + frequency + daterange;   due_date := d;   WHILE due_date + frequency <= max_date LOOP
--Don't include dates in the past - we only want future       -- due dates for bills.       IF due_date >= CURRENT_DATE
     THEN           RETURN NEXT due_date;       END IF;       due_date := due_date + frequency;   END LOOP;   RETURN;
--exit function
 


This appears to work properly:

SELECT next_bill_date( '2011-06-01', '2 weeks', '3 months' );next_bill_date
----------------2011-06-152011-06-292011-07-132011-07-272011-08-102011-08-242011-09-07
(7 rows)

Thanks for all your help!  I'm not at all experienced with plpgsql,
so this was very much appreciated.  :)

Benny


-- 
"You were doing well until everyone died."                                   -- "God", Futurama