ORA-00934: group function is not allowed here when adding case when - sql

Question

I have a piece of code which runs fine. However, when i am introducing a "case when" statement in the select clause, I get the "group function is not allowed here" error and I cannot fix it (the issue relates to the last Group by function in my code)
Any idea why (don't be put off by the code, it is 3 joins together, apparently the issue is caused by the last Group By statement) ?
Thank you!
SELECT
Trans_Table.MTAGRE01_NO
, (case when Cash. MTAGRE01_NO = Trans_Table. MTAGRE01_NO
then (SUM(Trans_Table.MTTRANS01_VALUENCU)*-1)
else SUM(Trans_Table.MTTRANS01_VALUENCU) END) AS MTTRANS01_VALUENCU
FROM MTTRANS01 Trans_Table
INNER JOIN RUTRANTYPE01 Trans_Type
ON Trans_Type.RUTRANTYPE01_CODE = Trans_Table.RUTRANTYPE01_CODE
LEFT JOIN(
SELECT
MTAGRE01_NO
,CASE WHEN SRAGRESTAT01_CODE = 'COLL' THEN MTAGRE01_AGRESTATDATE END AS Date_Fr
from MTAGRE01
where CASE WHEN SRAGRESTAT01_CODE = 'COLL' THEN MTAGRE01_AGRESTATDATE END is not null
) F_Date
ON F_Date.MTAGRE01_NO = Trans_Table.MTAGRE01_NO
LEFT JOIN(
SELECT
Trans_Table.MTAGRE01_NO
FROM MTTRANS01 Trans_Table
INNER JOIN RUTRANTYPE01 Trans_Type ON Trans_Type.RUTRANTYPE01_CODE = Trans_Table.RUTRANTYPE01_CODE
GROUP BY
Trans_Table.MTAGRE01_NO, Trans_Type.RUTRANTYPE01_CODE, Trans_Type.RUTRANTYPE01_DESCRIPTION
) Cash
ON Cash.MTAGRE01_NO = Trans_Table.MTAGRE01_NO
where Trans_Type.SRPROCTYPE01_CODE in ('C','D')
and Trans_Table.MTTRANS01_VALUEDATE >= F_Date.Date_Fr
GROUP BY
Trans_Table.MTAGRE01_NO
, (case when Cash. MTAGRE01_NO = Trans_Table. MTAGRE01_NO
then (SUM(Trans_Table.MTTRANS01_VALUENCU)*-1)
else SUM(Trans_Table.MTTRANS01_VALUENCU) END);

Answer 1

I believe it's hung up on the sum inside the case statement. 2 routes to correct that I can see, likely alot more:
This is a little hacky, but it'll work and give results fast. Change your case:
SELECT
Trans_Table.MTAGRE01_NO
, (case when Cash. MTAGRE01_NO = Trans_Table. MTAGRE01_NO
then ((Trans_Table.MTTRANS01_VALUENCU)*-1)
else (Trans_Table.MTTRANS01_VALUENCU) END) AS MTTRANS01_VALUENCU
Remove the case from the group by.
Now call your entire query a sub query
Select MTAGRE01_NO, sum(MTTRANS01_VALUENCU)
(your entire query)a
You other option that takes a bit more work...in your initial from statement:
MTTRANS01 Trans_Table
Change that to a subquery that joins to the trans table and returns
case when Cash. MTAGRE01_NO = Trans_Table. MTAGRE01_NO
then ((Trans_Table.MTTRANS01_VALUENCU)*-1)
else (Trans_Table.MTTRANS01_VALUENCU) END as MTAGRE01_NO
Then join to that subquery and do a simple sum in your main query.
Hopefully that all makes sense, ask questions if you need clarifications