Oracle ORA -932 expected Number got CHAR - oracle

Question

I am running the following query and can't seem to figure out where the error is-
select case when month>=7 then substr(year,3,2)|| '/'||TO_NUMBER( substr(year,3,2))+1
else to_number(substr(year,3,2))-1 || '/'|| substr(year,3,2) end as fiscal_year
FROM ( SELECT DISTINCT to_Char(extract( year from date)) as year,
extract( month from date)as MONTH FROM TABLE )
I want to convert year to fiscal year like 19/20, 20/21 etc

Answer 1

The operator precedence rules means the string concatenation is happening before the addition; this expression:
substr(year,3,2)|| '/'||TO_NUMBER( substr(year,3,2))+1
is evaluated as
substr(year,3,2)|| '/'||TO_NUMBER( substr(year,3,2))
and then it tries to add 1 to that string result. Hence the error you get.
You can add parentheses to make it add 1 to the year number before then concatenating that:
substr(year,3,2)|| '/'|| (TO_NUMBER( substr(year,3,2))+1)
You could also do this without so much string manipulation:
select case when extract (month from your_date) >= 7 then
to_char(your_date, 'YY') || '/' || to_char(add_months(your_date, 12), 'YY')
else
to_char(add_months(your_date, -12), 'YY') || '/' || to_char(your_date, 'YY')
end as fiscal_year
FROM (
SELECT DISTINCT trunc(your_date, 'MM') as your_date
FROM your_table
)
db<>fiddle
and there are other options, of course.