Rewrite PostgreSQL query using CTE: - sql

Question

I have the following code to pull records from a daterange in PostgreSQL, it works as intended. The "end date" is determined by the "date" column from the last record, and the "start date" is calculated by subtracting a 7-day interval from the "end date".
SELECT date
FROM files
WHERE daterange((
(SELECT date FROM files ORDER BY date DESC LIMIT 1) - interval '7 day')::date, -- "start date"
(SELECT date FROM files ORDER BY date DESC LIMIT 1)::date, -- "end date"
'(]') #> date::date
ORDER BY date ASC
I'm trying to rewrite this query using CTEs, so I can replace those subqueries with values such as end_date and start_date. Is this possible using this method or should I look for other alternatives like variables? I'm still learning SQL.
WITH end_date AS
(
SELECT date FROM files ORDER BY date DESC LIMIT 1
),
start_date AS
(
SELECT date FROM end_date - INTERVAL '7 day'
)
SELECT date
FROM files
WHERE daterange(
start_date::date,
end_date::date,
'(]') #> date::date
ORDER BY date ASC
Right now I'm getting the following error:
ERROR: syntax error at or near "-"
LINE 7: SELECT date FROM end_date - INTERVAL '7 day'

Answer 1

You do not need two CTEs, it's one just fine, which can be joined to filter data.
WITH RECURSIVE files AS (
SELECT CURRENT_DATE date, 1 some_value
UNION ALL
SELECT (date + interval '1 day')::date, some_value + 1 FROM files
WHERE date < (CURRENT_DATE + interval '1 month')::date
),
dates AS (
SELECT
(MAX(date) - interval '7 day')::date from_date,
MAX(date) to_date
FROM files
)
SELECT f.* FROM files f
JOIN dates d ON daterange(d.from_date, d.to_date, '(]') #> f.date
You even can make it to be a daterange initially in CTE and use it later like this
WITH dates AS (
SELECT
daterange((MAX(date) - interval '7 day')::date, MAX(date), '(]') range
FROM files
)
SELECT f.* FROM files f
JOIN dates d ON d.range #> f.date
Here the first CTE is used just to generate some data.
It will get all file lines for dates in the last week, excluding from_date and including to_date.
date
some_value
2022-09-26
25
2022-09-27
26
2022-09-28
27
2022-09-29
28
2022-09-30
29
2022-10-01
30
2022-10-02
31

Answer 2

I think this is what you want:
WITH end_date AS
(
SELECT date FROM files ORDER BY date DESC LIMIT 1
),
start_date AS
(
SELECT date - INTERVAL '7 day' as date
FROM end_date
)
SELECT F.date, S.date startDate, E.date endDate
FROM files F
JOIN start_date S on F.date >= S.date
JOIN end_date E on F.date <= E.date
ORDER BY date ASC;

Answer 3

I hope I'm not repeating anything, but if I understand your problem correctly I think this will work:
with cte as (
select max (date)::date as max_date from files
)
select date
from files
cross join cte
where date >= max_date - 7
Or perhaps even:
select date
from files
where date >= (select max (date)::date - 7 from files)
Since you have already determined that the CTE has the max date, there is really no need to further bound it with a between, <= or range. You can simply say anything after that date minus 7 days.
The error in your code above is because you want this:
SELECT date - INTERVAL '7 day' as date FROM end_date
And not this:
SELECT date FROM end_date - INTERVAL '7 day'
You are subtracting from the table, which doesn't make sense.