Aggregate yearly spend of user based on billing cycle (SQL) - sql

Question

I want to track the yearly spend of users based on a particular month from which we start their cycle. This is to keep track of their yearly spend so that they don't exceed the allowed limits. I have the following two tables:
Spend (Contains 1 row per user per month) (I can modify the date column of this table to any date format as needed, if it helps):
+----+-----------+------+-------+-------+
| ID | Date | Year | Month | Spend |
+----+-----------+------+-------+-------+
| 11 | 01-Sep-19 | 2019 | 9 | 10 |
+----+-----------+------+-------+-------+
| 11 | 01-Oct-19 | 2019 | 10 | 23 |
+----+-----------+------+-------+-------+
| 11 | 01-Nov-19 | 2019 | 11 | 27 |
+----+-----------+------+-------+-------+
| 11 | 01-Dec-19 | 2019 | 12 | 14 |
+----+-----------+------+-------+-------+
| 11 | 01-Jan-20 | 2020 | 1 | 13 |
+----+-----------+------+-------+-------+
| 11 | 01-Feb-20 | 2020 | 2 | 33 |
+----+-----------+------+-------+-------+
| 11 | 01-Mar-20 | 2020 | 3 | 25 |
+----+-----------+------+-------+-------+
| 11 | 01-Apr-20 | 2020 | 4 | 17 |
+----+-----------+------+-------+-------+
| 11 | 01-May-20 | 2020 | 5 | 14 |
+----+-----------+------+-------+-------+
| 11 | 01-Jun-20 | 2020 | 6 | 10 |
+----+-----------+------+-------+-------+
| 11 | 01-Jul-20 | 2020 | 7 | 46 |
+----+-----------+------+-------+-------+
| 11 | 01-Aug-20 | 2020 | 8 | 53 |
+----+-----------+------+-------+-------+
| 11 | 01-Sep-20 | 2020 | 9 | 38 |
+----+-----------+------+-------+-------+
| 11 | 01-Oct-20 | 2020 | 10 | 22 |
+----+-----------+------+-------+-------+
| 11 | 01-Nov-20 | 2020 | 11 | 29 |
+----+-----------+------+-------+-------+
| 50 | 01-Jul-20 | 2020 | 7 | 56 |
+----+-----------+------+-------+-------+
| 50 | 01-Aug-20 | 2020 | 8 | 62 |
+----+-----------+------+-------+-------+
| 50 | 01-Sep-20 | 2020 | 9 | 77 |
+----+-----------+------+-------+-------+
| 50 | 01-Oct-20 | 2020 | 10 | 52 |
+----+-----------+------+-------+-------+
| 50 | 01-Nov-20 | 2020 | 11 | 45 |
+----+-----------+------+-------+-------+
Billing Cycle (contains the months between which we calculate their total spends):
+-----+------------+----------+
| ID | StartMonth | EndMonth |
+-----+------------+----------+
| 11 | 10 | 9 |
+-----+------------+----------+
| 50 | 9 | 8 |
+-----+------------+----------+
Sample Output:
+----+-------+------------+
| ID | Cycle | TotalSpend |
+----+-------+------------+
| 11 | 1 | 10 |
+----+-------+------------+
| 11 | 2 | 313 |
+----+-------+------------+
| 11 | 3 | 51 |
+----+-------+------------+
| 50 | 1 | 118 |
+----+-------+------------+
| 50 | 2 | 174 |
+----+-------+------------+
In the sample output, for ID = 11, cycle 1 indicates spend in Sep'19, cycle 2 indicates total spend from Oct'19 (Month 10) to Sep'20 (Month 9) and cycle 3 indicates total spend for the next 12 months from Oct'20 (till whichever month data is present).
I'm a beginner to SQL and I believe doing this might require the use of CTE/Subqueries. Would appreciate any help or guidance for this.

Answer 1

Since this seems to be an exercise of some sort, I'm not going to provide a full answer, but give you hints to how this could be solved conceptually.
First I think you should associate entries to effective cycles (with cycle number) for the required date range. This could be done by using a recursive CTE. These are not the most efficient approach, but since we don't have the effective cycles with their numbers as a distinct table it can be a working solution nevertheless.
The result then just needs to be grouped by the ID and cycle number and the amounts summed up, and you're done.