Sum based on column condition

Question

I am trying to sum sales on my table grouped for next 30 days.

Here is the data I have.

date	sales
2021-08-08	35
2021-08-08	14
2021-08-11	35
2021-09-09	22
2021-09-21	44
2021-10-16	46
2021-10-25	9
2021-10-25	1
2021-10-25	2
2021-10-25	6
2021-11-04	1
2021-11-07	1

I am expecting the result like this

date	sales	total 30d
2021-08-08	14	84
2021-08-08	35	84
2021-08-11	35	57
2021-09-09	22	66
2021-09-21	44	90
2021-10-16	46	76
2021-10-25	9	30
2021-10-25	6	30
2021-10-25	2	30
2021-10-25	1	30
2021-11-04	1	12
2021-11-07	1	29

Explanation: For date 2021-08-08 it's going to sum the sales from date >= 2021-08-08 til 30 days after which is 2021-09-07 so it would be 14+35+35 (2021-08-08, 2021-08-08 and 2021-08-11).

I got my work around using this query, but I think it is very inefficient and I believe there's other way using windows function though I can't wrap my head around it.

with temp as (
    select date, sum(sales) as sales_total
from test_table
group by 1,2
)
, temp2 as (
SELECT a.date, SUM(b.sales_total) total
FROM temp a, temp b
WHERE b.date >= a.date AND b.date <= a.date + interval '30' day
GROUP BY a.date
)
select a.date, a.sales, b.total 
from test_table a 
JOIN temp2 b on a.date = b.date

Any pointer to solve this problem using windows function or any other more efficient way is appreciated. Thanks!

Answer 1

Here is a way to do this using recursive cte.

with data
   as (select start_dt,sales,row_number() over(order by start_dt) as rnk       
         from t1
       )
     ,cte_data
    as ( select d.start_dt,d.sales,d.rnk,d.start_dt as grp_dt
           from data d
          where rnk=1
        union all
        select d.start_dt,d.sales,d.rnk,case when datediff(day,d2.grp_dt,d.start_dt)>30 then 
                                           d.start_dt
                                     else d2.grp_dt
                                 end
          from cte_data d2
          join data d
            on d.rnk=d2.rnk+1
       )
  select min(start_dt) start_range,max(start_dt) end_range,sum(sales) as sum_sales
    from cte_data
 group by grp_dt

+-------------+------------+-----------+
| start_range | end_range  | sum_sales |
+-------------+------------+-----------+
| 2021-08-08  | 2021-08-11 |        84 |
| 2021-09-09  | 2021-09-21 |        66 |
| 2021-10-16  | 2021-11-07 |        66 |
+-------------+------------+-----------+

sql fiddle link https://dbfiddle.uk/2LmQe5n5

To elaborate a bit on this. We would need some kind of recursive process to do this, as the results of the following row depends on the previous entry date, and so I doubt we could do this using window functions.