Is it possible to add distinct to part of a sum clause in Oracle? - oracle

Question

I have a pretty lengthy SQL query which I'm going to run on Oracle via hibernate. It consists of two nested selects. In the first select statement, a number of sums are calculated, but in one of them I want to filter the results using unique ids.
SELECT ...
SUM(NVL(CASE WHEN SECOND_STATUS= 50 OR SECOND_STATUS IS NULL THEN RECEIVE_AMOUNT END, 0) +
NVL(CASE WHEN FIRST_STATUS = 1010 THEN AMOUNT END, 0) +
NVL(CASE WHEN FIRST_STATUS = 1030 THEN AMOUNT END, 0) -
NVL(CASE WHEN FIRST_STATUS = 1010 AND (SECOND_STATUS= 50 OR SECOND_STATUS IS NULL) THEN RECEIVE_AMOUNT END, 0)) TOTAL, ...
And at the end:
... FROM (SELECT s.*, p.* FROM FIRST_TABLE s
JOIN SECOND_TABLE p ON s.ID = p.FIRST_ID
In one of the lines that start with NVL (second line actually), I want to add a distinct clause that sums the amounts only if first table ids are unique. But I don't know if this is possible or not. If yes, how would it be?

Answer 1

Assume such setup
select * from first;
ID AMOUNT
---------- ----------
1 10
2 20
select * from second;
SECOND_ID FIRST_ID AMOUNT2
---------- ---------- ----------
1 1 100
2 1 100
3 2 100
After the join you get the total sum of both amounts too high because the amount from the first table is duplicated.
select *
from first
join second on first.id = second.first_id;
ID AMOUNT SECOND_ID FIRST_ID AMOUNT2
---------- ---------- ---------- ---------- ----------
1 10 1 1 100
1 10 2 1 100
2 20 3 2 100
You must add a row_number that identifies the first occurence in the parent table and consider in the AMOUNT only the first row and resets it to NULL in the duplicated rows.
select ID,
case when row_number() over (partition by id order by second_id) = 1 then AMOUNT end as AMOUNT,
SECOND_ID, FIRST_ID, AMOUNT2
from first
join second on first.id = second.first_id;
ID AMOUNT SECOND_ID FIRST_ID AMOUNT2
---------- ---------- ---------- ---------- ----------
1 10 1 1 100
1 2 1 100
2 20 3 2 100
Now you can safely sum in a separate subquery
with tab as (
select ID,
case when row_number() over (partition by id order by second_id) = 1 then AMOUNT end as AMOUNT,
SECOND_ID, FIRST_ID, AMOUNT2
from first
join second on first.id = second.first_id
)
select id, sum(nvl(amount,0) + nvl(amount2,0))
from tab
group by id
;
ID SUM(NVL(AMOUNT,0)+NVL(AMOUNT2,0))
---------- ---------------------------------
1 210
2 120
Note also that this is an answer to your question. Some will argue that in your situation you should first aggregate and than join. This will also resolve your problem possible more elegantly.