Replace Union All to join to improve performance - sql

Question

I have a working query which takes 20 mins to return data. I want to optimize it .
I have table
Incentives:
Transaction_ID | Incentive_On_A | Incentive_On_B | Incentive_On_C
--------------+-----------------+-----------------+---------------
1 | 0 | 0 | 10
2 | 30 | 0 | 0
3 | 0 | 20 | 0
4 | 40 | 0 | 0
Required Output:
Transaction_ID| Product_Category | Incentive_Amt
---------- + -----------------+--------------
1 | A | 30
2 | B | 20
3 | C | 10
4 | A | 40
I am using this query :
select Transaction_ID, 'A' as Product_Category,
Incentive_On_A from Incentives
Union all
select Transaction_ID, 'B' as Product_Category,
Incentive_On_B from Incentives
Union all
select Transaction_ID, 'C' as Product_Category,
Incentive_On_C from Incentives
Is there any way I can optimize this query by removing union all with join?
Thanks alot for the help.
Edited**
1.Added one more row in both the tables.
Note:- Basically we are just doing a transpose of data - converting columns- 'Incentive_on_A','Incentive_on_B','Incentive_on_C' to a column - 'Category' having the values of the above 3 columns.

Answer 1

You don't need a JOIN here, you just need to unpivot your data:
SELECT transaction_id, REGEXP_SUBSTR(incentive_col, '[^_]*$') AS product_category
, incentive_amt
FROM (
SELECT transaction_id, incentive_a, incentive_b, incentive_c
FROM incentives
) UNPIVOT (
incentive_amt
FOR incentive_col IN (incentive_a, incentive_b, incentive_c )
) WHERE incentive_amt > 0;
Whether or not this will actually improve your performance, I could not say. My guess is that with the UNION ALL version of your query you're actually doing a full table scan 3 times.

Answer 2

To start with: this is a bad datamodel. If each record can only have one value, then just store one value, exactly as shown in your desired output.
As is, you can just add all values and use CASE WHEN to see which value is greater than zero:
select
transaction_id,
case when incentive_on_a > 0 then 'A'
when incentive_on_b > 0 then 'B'
when incentive_on_c > 0 then 'C'
end as product_category,
incentive_on_a + incentive_on_b + incentive_on_c as incentive_amt
from incentives
order by transaction_id;
(However, I still fail to see how such simple query like the one you are showing can run twenty minutes.)