Database

select round(  (exp(sum(ln(x))))  ,0) as ProductExample

  from (

    SELECT 3 X FROM DUAL

    UNION ALL 

    SELECT 5 X FROM DUAL

    UNION ALL

    SELECT 2 X FROM DUAL)

Output: 

3

5

2

Output: 

30

The Oracle/PLSQL NULLIF function compares expr1 and expr2. If expr1 and expr2 are equal, the NULLIF function returns NULL. Otherwise, it returns expr1.

https://www.oracletutorial.com/oracle-basics/oracle-group-by/