SQL Group by CASE result - sql

Question

I have a simple SQL query on IBM DB2. I'm trying to run something as below:
select case when a.custID = 42285 then 'Credit' when a.unitID <> '' then 'Sales' when a.unitID = '' then 'Refund'
else a.unitID end TYPE, sum(a.value) as Total from transactions a
group by a.custID, a.unitID
This query runs, however I have a problem with group by a.custID - I'd prefer not to have this, but the query won't run unless it's present. I'd want to run the group by function based on the result of the CASE function, not the condition pool behind it. So, I'm looking something like:
group by TYPE
However adding group by TYPE reports an error message "Column or global variable TYPE not found". Also removing a.custID from group section reports "Column custID or expression in SELECT list not valid"
Is this going to be possible at all or do I need to review my CASE function and avoid using the custID column since at the moment I'm getting a grouping also based on custID column, even though it's not present in SELECT.
I understand why the grouping works as it does, I'm just wondering if it's possible to get rid of the custID grouping, but still maintain it within CASE function.

Answer 1

If you want terseness of code, you could use a subquery here:
SELECT TYPE, SUM(value) AS Total
FROM
(
SELECT CASE WHEN a.custID = 42285 THEN 'Credit'
WHEN a.unitID <> '' THEN 'Sales'
WHEN a.unitID = '' THEN 'Refund'
ELSE a.unitID END TYPE,
value
FROM transactions a
) t
GROUP BY TYPE;
The alternative to this would be to just repeat the CASE expression in the GROUP BY clause, which is ugly, but should still work. Note that some databases (e.g. MySQL) have overloaded GROUP BY and do allow aliases to be used in at least some cases.