Incorrect results from max and rank function - oracle

Question

I am trying to return a row with the latest psi score but my code returns multiple rows and the row that I actually want is returned as the last row on result set. What can I do with this code to resolve the issue.
S
ELECT REQUEST_NUM,PI_CANDIDATE_NUM,
COALESCE (MAX_GCO_AD_KNOWLEDGE_TEST_SCORE, MAX_PSI_OVERALL_SCORE) AS PSI_SCORE
--COALESCE(MAX_GCO_AD_KNOWLEDGE_TEST_SCORE,MAX_PSI_OVERALL_SCORE)-39.035024/8.439997 AS PSI XMation
FROM(
SELECT
REQUEST_NUM,PI_CANDIDATE_NUM,
MAX(AA.PSI_OVERALL_SCORE) KEEP (DENSE_RANK FIRST ORDER BY AA.ARANK) MAX_PSI_OVERALL_SCORE,
MAX(AA.GCO_AD_KNOWLEDGE_TEST_SCORE) KEEP (DENSE_RANK FIRST ORDER BY AA.ARANK) MAX_GCO_AD_KNOWLEDGE_TEST_SCORE
FROM (
select
RANK() OVER (PARTITION BY REQUEST_NUM ORDER BY REQUEST_LAST_MODIFIED_DT ) ARANK
,PARENT_PI_NUMBER,REQUEST_NUM,PI_CANDIDATE_NUM,PSI_OVERALL_SCORE,GCO_AD_KNOWLEDGE_TEST_SCORE,REQUEST_LAST_MODIFIED_DT
from
WC_APPLICATION_EVENT_F
-- GCO_AD_KNOWLEDGE_TEST_SCORE != '10100' and
where PI_CANDIDATE_NUM = '4173093'
--nd GCO_AD_KNOWLEDGE_TEST_SCORE is null
) AA
--where AA.ARANK=1
GROUP BY REQUEST_NUM,PI_CANDIDATE_NUM
--ORDER By PARENT_PI_NUMBER,PI_NUMBER,REQUEST_LAST_MODIFIED_DT;
)
BB;
Sample data:
REQUEST_NUM PI_CANDIDATE_NUM REQUEST_LAST_MODIFIED_DT PSI_SCORE
----------- ---------------- ------------------------ ---------
4639022
1655626 4639022 5-Mar-17
1662401 4639022 8-Mar-17 22.6
1662470 4639022 6-Apr-17
1662486 4639022 6-Apr-17
1662499 4639022 8-Mar-17 30.3
1771817 4639022 7-Jun-17 35.3
1797323 4639022 24-Jun-17 38.5
My expected results is the last row with a value of 38.5 since has the latest date.

Answer 1

Here is one way to do this. It assumes you input a specific candidate (otherwise the analytic function in the subquery can be modified to get the most recent NON-NULL score for each candidate).
EDIT: If you need to retrieve the most recent score for ALL candidates, then: (1) remove the condition that filters on a single candidate in the inner query; (2) add partition by pi_candidate_num right after rank() over ( (after the opening parenthesis) and add a space before order by ... (still in the over(.....) clause of rank()). \EDIT
If two NON-NULL scores were achieved ON THE SAME DATE (perhaps that is impossible in your data, but if it is...) then BOTH rows will be returned.
I assumed the candidate number is a NUMBER, so I removed the single quotes in the condition; and I changed the candidate number to match your sample inputs.
The WITH clause is NOT PART OF THE QUERY - remove it before you try the solution.
with
test_data ( request_num, pi_candidate_num, request_last_modified_dt, psi_score ) as (
select null, 4639022, null , null from dual union all
select 1655626, 4639022, to_date( '5-Mar-17', 'dd-Mon-rr'), null from dual union all
select 1662401, 4639022, to_date( '8-Mar-17', 'dd-Mon-rr'), 22.6 from dual union all
select 1662470, 4639022, to_date( '6-Apr-17', 'dd-Mon-rr'), null from dual union all
select 1662486, 4639022, to_date( '6-Apr-17', 'dd-Mon-rr'), null from dual union all
select 1662499, 4639022, to_date( '8-Mar-17', 'dd-Mon-rr'), 30.3 from dual union all
select 1771817, 4639022, to_date( '7-Jun-17', 'dd-Mon-rr'), 35.3 from dual union all
select 1797323, 4639022, to_date('24-Jun-17', 'dd-Mon-rr'), 38.5 from dual
)
select request_num, pi_candidate_num, request_last_modified_dt, psi_score
from (
select t.*, rank() over (order by request_last_modified_dt desc) rn
from test_data t
where pi_candidate_num = 4639022
and psi_score is not null
)
where rn = 1
;
REQUEST_NUM PI_CANDIDATE_NUM REQUEST_LAST_MODIFIED_DT PSI_SCORE
----------- ---------------- ------------------------ ----------
1797323 4639022 24-Jun-17 38.5