Query to get the sum of values for the maximum number of months - sql

Question

This query in Oracle 11 gets the sum of value for the last 1 years, and it works when there are 1 years of data.
When there is less than 1 years of data, this query returns 0, instead of the sum of values until whatever the oldest years are.
For example, if there are only 6 months of data, the query should return the sum of values until the 6th month.
SELECT SUM (DECODE (rnk, 11, rt, 0)) 1Y
FROM (SELECT entity_id,rnk,
SUM (ABS(NVL (value, 0))) OVER (PARTITION BY TRIM (entity_id) ORDER BY rnk) rt
FROM (SELECT psm.*,RANK () OVER (PARTITION BY entity_id ORDER BY period_end_date DESC) AS rnk
FROM myTable psm
WHERE psm.entity_id = '1'
ORDER BY period_end_date DESC
) rank_tab
WHERE rnk < 12
);
If the biggest rank is 6, the result from the above query is 0
I attempted this, but got the error "ORA-00978: nested group function without GROUP BY"
SELECT case when rnk < 11
then SUM (DECODE (rnk, Max(rnk), rt, 0))
else SUM (DECODE (rnk, 11, rt, 0))
end as Y
FROM (SELECT entity_id,rnk,
SUM (ABS(NVL (value, 0))) OVER (PARTITION BY TRIM (entity_id) ORDER BY rnk) rt
FROM (SELECT psm.*,RANK () OVER (PARTITION BY entity_id ORDER BY period_end_date DESC) AS rnk
FROM myTable psm
WHERE psm.entity_id = '1'
ORDER BY period_end_date DESC
) rank_tab
WHERE rnk < 12
);
Sample data:
entity_id value period_end_date
1 1 9/30/19
1 2 8/31/19
1 3 7/31/19
1 4 6/30/19
1 5 5/31/19
1 6 4/30/19
In the above example, 1Y should return 1+2+3+4+5+6 = 21.
Instead my query returns 0 because it is looking for rnk = 11, which doesn't exist.
SUM (DECODE (rnk, 11, rt, 0)) 1Y
Thank you.
EDIT:
This works. But, if you know of a better way to do it, please let me know. Thank you.
SELECT
CASE WHEN MRank < 11 then maxY else OneY end as lc_incearned_1Y
FROM (
WITH R as
(SELECT MAX(RNK) MaxRank FROM (
SELECT RANK () OVER (PARTITION BY TRIM (entity_id) ORDER BY period_end_date
DESC) AS rnk FROM myTbl psm
WHERE TRIM (psm.entity_id) = '1' AND period_end_date <
to_date('9/30/2019','MM/DD/YYYY')
ORDER BY period_end_date DESC))
select MAX(MaxRank) MRank,
SUM (DECODE (rnk, MaxRank, rt, 0)) maxY,
SUM (DECODE (rnk, 11, rt, 0)) OneY, --13051.97
FROM (SELECT entity_id,rnk,
SUM (ABS (NVL (value, 0))) OVER (PARTITION BY TRIM (entity_id) ORDER BY rnk) rt
FROM (SELECT psm.*,RANK () OVER (PARTITION BY TRIM (entity_id) ORDER BY period_end_date DESC) AS rnk FROM CREF.PORTFOLIO_SUMM_MTHEND psm
WHERE TRIM (psm.entity_id) = '1' AND period_end_date < to_date('9/30/2019','MM/DD/YYYY')
ORDER BY period_end_date DESC) rank_tab WHERE rnk < 12) T,R)

Answer 1

It seems you need to sum your values up starting from the latest period_end_date to the earliest date within the eleven months range. It would be suitable to use max(period_end_date) over (partition by entity_id order by period_end_date desc) analytic function along with your current rank() function. And then apply months_between(<max_period_end_date>,period_end_date). If your need to look up from the current date, then get rid of max() analytic function and replace <max_period_end_date> with trunc(sysdate) in months_between() function. So, use :
with t as
(
select max(period_end_date) over (partition by entity_id order by period_end_date desc) as mx,
rank() over (partition by entity_id order by period_end_date desc) as rnk,
t.*
from myTable t
)
select sum(nvl(value,0)) as sum_value
from t
where months_between(mx,period_end_date)<=11
Demo