use < and > rather than BETWEEN in order to eliminate any overlap between conditions : BETWEEN « PL SQL Operators « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. PL SQL Operators
3. BETWEEN

SQL>
SQL> DECLARE
  2    salary NUMBER := 20000;
  3    employee_id NUMBER := 36325;
  4
  5    PROCEDURE give_bonus (emp_id IN NUMBER, bonus_amt IN NUMBER) IS
  6    BEGIN
  7      DBMS_OUTPUT.PUT_LINE(emp_id);
  8      DBMS_OUTPUT.PUT_LINE(bonus_amt);
  9    END;
 10
 11  BEGIN
 12  IF salary >= 10000 AND salary <= 20000
 13  THEN
 14     give_bonus(employee_id, 1500);
 15  ELSIF salary > 20000 AND salary <= 40000
 16  THEN
 17     give_bonus(employee_id, 1000);
 18  ELSIF salary > 40000
 19  THEN
 20     give_bonus(employee_id, 500);
 21  END IF;
 22  END;
 23  /

PL/SQL procedure successfully completed.