Function in Oracle SQL where clause

Question

I have a query like this:

SELECT * 
FROM employees emp 
WHERE emp.loc = 'US' 
  AND usr.fnctn('emp_sal', :sal1, :sal2);

This query causes an error:

ORA-00920: invalid RELATIONAL OPERATOR

My function just returns the highest and the lowest sal in employees table

Answer 1

A function returns a value.

In the where clause you need to compare that return value with something, eg

SELECT * 
FROM employees emp 
WHERE emp.loc = 'US' 
AND usr.fnctn('emp_sal', :sal1, :sal2) = 42;