function AFormatTrigger return boolean is
begin
  return (:P_A IS NOT NULL); 
end;

SQL> select
  2    decode('&p_a', 'a', to_char(deptno),
  3                   'b', dname
  4          ) result
  5  from dept;

Enter value for p_a: a
old   2:   decode('&p_a', 'a', to_char(deptno),
new   2:   decode('a', 'a', to_char(deptno),

RESULT
----------------------------------------
10
20
30
40

SQL> /
Enter value for p_a: b
old   2:   decode('&p_a', 'a', to_char(deptno),
new   2:   decode('b', 'a', to_char(deptno),

RESULT
----------------------------------------
ACCOUNTING
RESEARCH
SALES
OPERATIONS

SQL>

<snip>
   retval some_table.col_A%rowtype;
BEGIN
   IF :p_a = 'a' 
   THEN
      SELECT col_A into :retval
      FROM some_table
      WHERE ...;
   ELSIF :p_a = 'b' 
   THEN
      SELECT col_B into :retval
      FROM some_table
      WHERE ...;
   ELSIF
   ...
   END IF;

   RETURN (retval);
</snip>