Using function in SQL Query

[Jilly777]

I know nothing. I am trying to change an existing report. The earlier version used a function in a package in an SQL Query. For example:

SELECT A.FOO, DB.PKG.TO_STR(A.ID) AS NUM
FROM DB.TBL A
WHERE FOO >= _start
AND FOO <= _end
ORDER BY FOO

Now, I want to replace the function in the package. I thought I could just put the function in the Program Units (let's say it is MY_FUNC) and call it like:

SELECT A.FOO, MY_FUNC(A.ID) AS NUM
FROM DB.TBL A
WHERE FOO >= _start
AND FOO <= _end
ORDER BY FOO

But, it says MY_FUNC is an invalid identifier. How can I use a function inside the report in the Queries section?