How to select a single row for each unique ID - sql

Question

SQL novice here learning on the job, still a greenhorn. I have a problem I don't know how to overcome. Using IBM Netezza and Aginity Workbench.
My current output will try to return one row per case number based on when a task was created. It will only keep the row with the newest task. This gets me about 85% of the way there. The issue is that sometimes multiple tasks have a create day of the same day.
I would like to incorporate Task Followup Date to only keep the newest row if there are multiple rows with the same Case Number. I posted an example of what my current code outputs and what i would like it to output.
Current code
SELECT
A.PS_CASE_ID AS Case_Number
,D.CASE_TASK_TYPE_NM AS Task
,C.TASK_CRTE_TMS
,C.TASK_FLWUP_DT AS Task_Followup_Date
FROM VW_CC_CASE A
INNER JOIN VW_CASE_TASK C ON (A.CASE_ID = C.CASE_ID)
INNER JOIN VW_CASE_TASK_TYPE D ON (C.CASE_TASK_TYPE_ID = D.CASE_TASK_TYPE_ID)
INNER JOIN ADMIN.VW_RSN_CTGY B ON (A.RSN_CTGY_ID = B.RSN_CTGY_ID)
WHERE
(A.PS_Z_SPSR_ID LIKE '%EFT' OR A.PS_Z_SPSR_ID LIKE '%CRDT')
AND CAST(A.CASE_CRTE_TMS AS DATE) >= '2020-01-01'
AND B.RSN_CTGY_NM = 'Chargeback Initiation'
AND CAST(C.TASK_CRTE_TMS AS DATE) = (SELECT MAX(CAST(C2.TASK_CRTE_TMS AS DATE)) from VW_CASE_TASK C2 WHERE C2.CASE_ID = C.CASE_ID)
GROUP BY
A.PS_CASE_ID
,D.CASE_TASK_TYPE_NM
,C.TASK_CRTE_TMS
,C.TASK_FLWUP_DT
Current output
Desired output

Answer 1

You could use ROW_NUMBER here:
WITH cte AS (
SELECT DISTINCT A.PS_CASE_ID AS Case_Number, D.CASE_TASK_TYPE_NM AS Task,
C.TASK_CRTE_TMS, C.TASK_FLWUP_DT AS Task_Followup_Date,
ROW_NUMBER() OVER (PARTITION BY A.PS_CASE_ID ORDER BY C.TASK_FLWUP_DT DESC) rn
FROM VW_CC_CASE A
INNER JOIN VW_CASE_TASK C ON A.CASE_ID = C.CASE_ID
INNER JOIN VW_CASE_TASK_TYPE D ON C.CASE_TASK_TYPE_ID = D.CASE_TASK_TYPE_ID
INNER JOIN ADMIN.VW_RSN_CTGY B ON A.RSN_CTGY_ID = B.RSN_CTGY_ID
WHERE (A.PS_Z_SPSR_ID LIKE '%EFT' OR A.PS_Z_SPSR_ID LIKE '%CRDT') AND
CAST(A.CASE_CRTE_TMS AS DATE) >= '2020-01-01' AND
B.RSN_CTGY_NM = 'Chargeback Initiation' AND
CAST(C.TASK_CRTE_TMS AS DATE) = (SELECT MAX(CAST(C2.TASK_CRTE_TMS AS DATE))
FROM VW_CASE_TASK C2
WHERE C2.CASE_ID = C.CASE_ID)
)
SELECT
Case_Number,
Task,
TASK_CRTE_TMS,
Task_Followup_Date
FROM cte
WHERE rn = 1;

Answer 2

One method used window functions:
with cte as (
< your query here >
)
select x.*
from (select cte.*,
row_number() over (partition by case_number, Task_Followup_Date
order by TASK_CRTE_TMS asc
) as seqnum
from cte
) x
where seqnum = 1;