Use IF/ELSIF/ELSE to verify the input parameter : Parameters « Function Procedure Packages « Oracle PL/SQL Tutorial

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2. Function Procedure Packages
3. Parameters

SQL> set serveroutput on
SQL> set echo on
SQL>
SQL> CREATE OR REPLACE FUNCTION emptype (paytype CHAR)
  2       RETURN VARCHAR2 IS
  3  BEGIN
  4       IF paytype = 'H' THEN
  5            RETURN 'Hourly';
  6       ELSIF paytype = 'S' THEN
  7            RETURN 'Salaried';
  8       ELSIF paytype = 'E' THEN
  9            RETURN 'Executive';
 10       ELSE
 11            RETURN 'Invalid Type';
 12       END IF;
 13  EXCEPTION
 14       WHEN OTHERS THEN
 15            RETURN 'Error Encountered';
 16  END emptype;
 17  /

Function created.

SQL>
SQL> BEGIN
  2       DBMS_OUTPUT.PUT_LINE('emptype ' || emptype('H'));
  3       DBMS_OUTPUT.PUT_LINE('emptype ' || emptype('S'));
  4       DBMS_OUTPUT.PUT_LINE('emptype ' || emptype('E'));
  5       DBMS_OUTPUT.PUT_LINE('emptype ' || emptype('A'));
  6  END;
  7  /
emptype Hourly
emptype Salaried
emptype Executive
emptype Invalid Type

PL/SQL procedure successfully completed.

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