PGSQL query to get a list of sequential dates from today - sql

Question

I am having an calendar table where I have added the list of dates on which no action should be performed.
The table is as follows and the date format is YYYY-MM-DD
date
2021-01-01
2021-04-05
2021-04-06
2021-04-07
2021-08-10
2021-11-22
2021-11-23
2021-11-24
2021-12-25
2021-12-31
Considering today is 2021-11-24.
The expected output is
date
2021-11-24
2021-11-23
2021-11-22
And Considering today is 2021-12-25
then the expected output is
date
2021-12-25
And Considering today is 2021-12-27
then the output should contain no data.
date
It should get me the sequence with today's date in descending order without a break of sequence.
I searched on various posts I did find some of the posts related to my question but the query was little complex with nested subqueries. Is there a way to achieve the output in a more optimized way. I am new to pgsql.

Answer 1

Create example table:
CREATE TABLE calendar (d date);
INSERT INTO calendar VALUES ('2021-11-23'),('2021-11-20');
Query:
SELECT * FROM
(SELECT CURRENT_DATE - '1 day'::interval * generate_series(0,10) AS d) a
LEFT JOIN calendar c ON (c.d=a.d);
a.d | c.d
---------------------+------------
2021-11-14 00:00:00 | Null
2021-11-15 00:00:00 | Null
2021-11-16 00:00:00 | Null
2021-11-17 00:00:00 | Null
2021-11-18 00:00:00 | Null
2021-11-19 00:00:00 | Null
2021-11-20 00:00:00 | 2021-11-20
2021-11-21 00:00:00 | Null
2021-11-22 00:00:00 | Null
2021-11-23 00:00:00 | 2021-11-23
2021-11-24 00:00:00 | Null
Subquery "a" generates a date series, and then we join it to the table.
You can add conditions , for example "WHERE calendar.d IS NULL", or "IS NOT NULL" depending on the filtering you want.

Answer 2

You can simply filter by a date range, building it by subtracting 2 days from today:
select "date"
from maintenance_dates_70099898
where "date" <= now()::date --you want to see today and 2 days prior; Last 3 days total
and "date" >= now()::date - '2 days'::interval
order by 1 desc;
With a runnable test:
drop table if exists maintenance_dates_70099898;
create table maintenance_dates_70099898 ("date" date);
insert into maintenance_dates_70099898
("date")
values
('2021-01-01'),
('2021-04-05'),
('2021-04-06'),
('2021-04-07'),
('2021-08-10'),
('2021-11-22'),
('2021-11-23'),
('2021-11-24'),
('2021-12-25'),
('2021-12-31');
select "date"
from maintenance_dates_70099898
where "date" <= now()::date --you want to see today and 2 days prior; Last 3 days total
and "date" >= now()::date - '2 days'::interval
order by 1 desc;
-- date
--------------
-- 2021-11-24
-- 2021-11-23
-- 2021-11-22
--(3 rows)
select "date"
from maintenance_dates_70099898
where "date" >= '2021-12-25'::date - '2 days'::interval
and "date" <= '2021-12-25'::date
order by 1 desc;
-- date
--------------
-- 2021-12-25
--(1 row)
I assume that for 2021-12-27 you do want to see 2021-12-25, as it's within the 3 day range prior.
select "date"
from maintenance_dates_70099898
where "date" >= '2021-12-28'::date - '2 days'::interval
and "date" <= '2021-12-28'::date
order by 1 desc;
-- date
--------
--(0 rows)

Answer 3

The main issue appears to be not having a known number of days thus disabling a simple range validation/selection. However to the rescue there is a RECURSIVE cte to pluck off each previous date that is exactly 1 day prior to the last and terminate when no longer holds.
with recursive no_action(no_act_dt) as
( select no_act_dt
from no_action_calendar
where no_act_dt = :parm_date::date
union all
select c.no_act_dt
from no_action_calendar c
join no_action a
on (c.no_act_dt = a.no_act_dt - 1)
)
select *
from no_action
order by no_act_dt desc;
If you use this often or from several points, you can parametrize it with a SQL function. (see demo for both).
create or replace
function consective_no_action_dates (date_in date)
returns setof date
language sql
as $$
with recursive no_action(no_act_dt) as
( select no_act_dt
from no_action_calendar
where no_act_dt = date_in
union all
select c.no_act_dt
from no_action_calendar c
join no_action a
on (c.no_act_dt = a.no_act_dt - 1)
)
select *
from no_action
order by no_act_dt desc;
$$;