How do I use SQL window to sum rows with a condition - sql

Question

Assume this is my table:
id start_date event_date sales
------------------------------------
1 2020-09-09 2020-08-30 27.9
1 2020-09-09 2020-09-01 15
1 2020-09-09 2020-09-05 25
1 2020-09-09 2020-09-06 20.75
2 2020-09-09 2020-01-30 5
2 2020-09-09 2020-08-01 12
I'm trying to use a window function, where I want to sum sales in event_date for 7 days prior to the start date for each id, so the output I'm trying to reach looks like this...
id start_date event_date sales sales_7_days
-------------------------------------------------
1 2020-09-09 2020-08-30 27.9 0
1 2020-09-09 2020-09-01 15 0 <---- this is not within 7 days of start_date
1 2020-09-09 2020-09-05 25 25 <---- this is within 7 days of start_date
1 2020-09-09 2020-09-06 20.75 40.75 <---- this is within 7 days of start_date
2 2020-09-09 2020-01-30 5 0
2 2020-09-09 2020-09-03 12 12
This is what I've tried so far, but the problem is it seems to start summing from 7 days previous to event_date rather than start_date.
SELECT
id,
start_date,
event_date,
sales,
CASE WHEN event_date >= DATE_ADD(start_date, -7) THEN SUM(sales) \
OVER(PARTITION BY id ORDER BY event_date RANGE BETWEEN INTERVAL 7 DAYS PRECEDING AND CURRENT ROW) ELSE 0 END AS sales_7_days
FROM
sample_df
ORDER BY
id,
start_date,
event_date
So the query above is producing the below (which I don't want, because the window sum starts from event_date rather than start_date)
id start_date event_date sales sales_7_days
-------------------------------------------------
1 2020-09-09 2020-08-30 27.9 0
1 2020-09-09 2020-09-01 15 0
1 2020-09-09 2020-09-05 25 67.9
1 2020-09-09 2020-09-06 20.75 60.75
2 2020-09-09 2020-01-30 5 0
2 2020-09-09 2020-09-03 12 17
Does anybody have any tips here?

Answer 1

where I want to sum sales in event_date for 7 days prior to the start date for each id
Because the start date is constant for each id, this is a constant. You can calculate it as:
select s.*,
sum(case when event_date <= start_date and event_date >= start_date - interval 7 day
then sales
end) over (partition by id)
from sample_df s;
Your results suggest, though, that you really want a cumulative sum based on the event_date. That's fine, but a different question. The answer for that is to tweak the SQL:
select s.*,
sum(case when event_date <= start_date and event_date >= start_date - interval 7 day
then sales
end) over (partition by id order by event_date)
from sample_df s;