How to do a x-days grouped sum in redshift? - sql

Question

I have the following table,
that shows how many items from different units entered the inventory, in different dates.
ID Date Unit Quantity
---------------------------------
1 2017-08-01 A_red 05
2 2017-08-13 A_red 10
3 2017-09-20 A_red 20
4 2017-09-22 A_red 40
5 2017-10-05 A_red 40
6 2017-10-25 A_red 30
7 2017-10-24 A_blue 60
The problem is: entries within a time interval of 30 days of the same unit should be grouped.
So I want the following result:
ID Date Unit Quantity fst_entry30 Quantity30
-----------------------------------------------------
1 2017-08-01 A_red 05 T 15
2 2017-08-13 A_red 10 F 15
3 2017-09-20 A_red 20 T 100
4 2017-09-22 A_red 40 F 100
5 2017-10-05 A_red 40 F 100
6 2017-10-25 A_red 30 T 30
7 2017-10-24 A_blue 60 T 60
where fst_entry30 is a flag that points if the entry was the first, of this unit, in the last 30 days. Note that if i have a different unit (A_blue instead of A_red), it won't be grouped.
And quantity30 is the grouped sum of quantity.
For example, between 5 october and 20 september there is less than 30 days, so it was grouped.
Remembering that Redshift does not allow recursive common table expressions.
I already tried self-joins, but that turned out to be cumbersome.

Answer 1

You would just use lag() to define the groups:
select t.*,
(case when date >= lag(date) over (partition by unit order by date) + interval '30 day'
then 0 else 1
end) as grp_start
from t;
Then you can do a cumulative sum to assign a number to the group . . . and finally add them up using a window function:
select t.*, sum(quantity) over (partition by unit, grp)
from (select t.*,
sum(grp_start) over (partition by unit order by date) as grp
from (select t.*,
(case when date >= lag(date) over (partition by unit order by date) + interval '30 day'
then 0 else 1
end) as grp_start
from t
) t
) t