How to select a certain future date based on integer and integer[] in Postgresql? - postgresql

Question

I am trying to create a query that will return information about a series of future dates. So for example, today is Monday, and I want to get three days worth of information in advance: Tuesday, Wednesday, and Thursday. I understand how to use something like generate_series with a starting and end date to get the rows.
The problem I'm having is, I am selecting an integer for the number of days in advance I want from one table from a second table. But the particular dates will change if one or more of the potential future dates is one where the business is not open. So if the starting date were Thursday, and the business is closed on Sunday, I'd want to get rows for Friday, Saturday, and Monday.
So from the first table with the specifics on which days to get, I'd be selecting an integer (e.g. 3) and an integer[] (e.g. {1,2,3,4,5,6}). My thought was to somehow start with the day of the week of tomorrow (e.g. 2 from SELECT EXTRACT(DOW FROM CURRENT_DATE + '1 days'::interval)if today is tomorrow is Tuesday) and then check if that DOW is inside the array. I'd have a separate counter with the number of extra days I'd need to add to my series, and after looping through until I get three days that aren't skipped, I'd add it to my days ahead number. So starting on Thursday, I'd check Friday (5), it's in the array, increment loop variable and continue. Saturday (6), it's in the array, increment loop variable and continue. Sunday (0), not in the array, add one to the extra days counter and continue. Monday (1), in the array, increment loop variable and continue. That's three, so I'm done. Then add my second counter (1) to the original days ahead (3) and get 4 days worth of information. Days that the business isn't open will be excluded through WHERE conditions, so the total number of days displayed will be consistent.
The problem is, I can conceptualize this solution, but I can't figure out how to put it together syntactically. Here's an approximation of what I think would work:
DO $$
BEGIN
DECLARE
counter integer := 0;
increment_days integer := 1;
WITH future_data AS
(SELECT days_ahead, open_days FROM Stores);
WHILE counter < (SELECT days_ahead FROM future_data) loop
CASE WHEN (SELECT EXTRACT(DOW FROM CURRENT_DATE + (days::text || ' days'::interval))
= ANY(SELECT unnest(open_days) FROM future_data)) THEN
counter := counter + 1;
ELSE counter := counter END;
increment_days := increment_days + 1;
END LOOP;
increment_days := increment_days + days_ahead;
--[...main SELECT query...]
END$$;
I keep getting complains about the way I'm putting this all together. Currently it's a syntax error at WHILE. It seems like I can't do anything but a SELECT statement there.

Answer 1

Rather the trying to figure out how many days in advance just build a function where you provide a start_date and the number of days you want. Then let the function determine the actual dates returned (ie it bypasses Sunday). The following SQL function does that using a recursive CTE rather than attempting to calculate the number of days to look forward. See fiddle
create or replace
function business_day(start_date_in date, num_days_in integer default 3)
returns setof date
language sql
immutable strict
as $$
with recursive get_days (bus_date, num_selected) as
( select case when extract(dow from start_date_in::timestamp) > 0
then start_date_in::timestamp + interval '1 day'
else start_date_in::timestamp + interval '2 day'
end
, 1
union all
select case when extract(dow from bus_date + interval '1 day')>0
then bus_date + interval '1 day'
else bus_date + interval '2 day'
end
, num_selected + 1
from get_days
where num_selected<num_days_in
)
select bus_date::date from get_days ;
$$;