Calculating Datediff of two days based on when the sum of a column hits a number cap - sql

Question

Tried to see if this was asked anywhere else but doesn't seem like it. Trying to create a sql query to give me the date difference in days between '2022-10-01' and the date when our impression sum hits our cap of 5.
For context, we may see duplicate dates because someone revisit our website that day so we'll get a different session number to pair with that count. Here's an example table of one individual and how many impressions logged.
My goal is to get the number of days it takes to hit an impression cap of 5. So for this individual, they would hit the cap on '2022-10-07' and the days between '2022-10-01' and '2022-10-07' is 6. I am also calculating the difference before/after '2023-01-01' since I need this count for Q4 of '22 and Q1 of '23 but will not include in the example table. I have other individuals to include but for the purpose of asking here, I kept it to one.
Current Query:
select
click_date,
case
when date(click_date) < date('2023-01-01') and sum(impression_cnt = 5) then datediff('day', '2022-10-01', click_date)
when date(click_date) >= date('2023-01-01') and sum(impression_cnt = 5) then datediff('day', '2023-01-01', click_date)
else 0
end days_to_capped
from table
group by customer, click_date, impression_cnt
customer
click date
impression_cnt
123456
2022-10-05
2
123456
2022-10-05
1
123456
2022-10-06
1
123456
2022-10-07
1
123456
2022-10-11
1
123456
2022-10-11
3
Result Table
customer
days_to_cap
123456
6
I'm currently only getting 0 days and then 81 days once it hits 2022-12-21 (last date) for this individual so i know I need to fix my query. Any help would be appreciated!
Edited: This is in snowflake!

Answer 1

So, the issue with your query is that the sum is being calculated at the level that you are grouping by, which is every field, so it will always just be the value of the impressions field every time.
What you need to do is a running sum, which is a SUM() OVER (PARTITION BY...) statement. And then qualify the results of that:
First, just to get the data that you have:
with x as (
select *
from values
(123456,'2022-10-05'::date,2),
(123456,'2022-10-05'::date,1),
(123456,'2022-10-06'::date,1),
(123456,'2022-10-07'::date,1),
(123456,'2022-10-11'::date,1),
(123456,'2022-10-11'::date,3) x (customer,click_date,impression_cnt)
)
Then, I query the CTE to do the running sum with a QUALIFY statement to choose the record that actually has the value I'm looking for
select
customer,
case
when click_date < '2023-01-01'::date and sum(impression_cnt) OVER (partition by customer order by click_date) = 5 then datediff('day', '2022-10-01', click_date)
when click_date >= '2023-01-01'::date and sum(impression_cnt) OVER (partition by customer order by click_date) = 5 then datediff('day', '2023-01-01', click_date)
else 0
end days_to_capped
from x
qualify days_to_capped > 0;
The qualify filters your results to just the record that you cared about.