plsql subtract columns in a same table - sql

Question

I have a simple student table.
name | amount | vdate
Josh | 15 | 01.01.2020
Steve | 25 | 05.04.2008
Josh | 40 | 01.01.2022
What I want to do is subtract Josh value from each other.
I wrote this query but it is not working
select name , sum(b.amount-a.amount) diff from
select name,amount from student a where name = 'Josh' and vdate='01.01.2020'
union all
select name,amount from student b where name = 'Josh' and vdate = '01.01.2022')
group by name
Expected Result is:
name | diff
Josh | 25
Steve| 25

Answer 1

you can try this code,
select
fname,
abs(sum(amount2)) amount
from
(
WITH
student(fname,amount,vdate) AS (
SELECT 'Josh' ,15, to_date('01102017','ddmmyyyy') from dual
UNION ALL SELECT 'Steve',25, to_date('01102017','ddmmyyyy') from dual
UNION ALL SELECT 'Josh' ,40 ,to_date('01102019','ddmmyyyy')from dual
)
select
h.fname,
h.amount,
decode((ROW_NUMBER() OVER(PARTITION BY fname order by vdate desc)),1,amount,amount* -1) amount2
from student h
)
group by
fname
;
I assume that you get the greater amount value of the person and substract other values, you can select the bigger date instead by modifying the order by clause in the partition window i. e.
decode((ROW_NUMBER() OVER(PARTITION BY fname order by vdate desc)),1,amount,amount * -1) amount2

Answer 2

You can try this (I don't know what sense it makes ...):
Count the number of rows found until now per fname ("name" is a reserved word and I don't use it). And if the row number obtained this way is odd, then use the negative amount, else the positive amount.
Finally, run a sum over these positive/negative rows.
WITH
indata(fname,amount) AS (
SELECT 'Josh' ,15
UNION ALL SELECT 'Steve',25
UNION ALL SELECT 'Josh' ,40
)
,
alternate AS (
SELECT
fname
, CASE ROW_NUMBER() OVER(PARTITION BY fname) % 2
WHEN 1 THEN amount * -1 -- when odd then negative
ELSE amount -- else positive
END AS amount
FROM indata
)
SELECT
fname
, ABS(SUM(amount)) AS amount -- absolute value
FROM alternate
GROUP BY fname;
-- out fname | amount
-- out -------+--------
-- out Josh | 25
-- out Steve | 25