DECLARE
   a number;
   b number;
   c number;
FUNCTION findMax(x IN number, y IN number) 
RETURN number
IS
    z number;
BEGIN
   IF x > y THEN
      z:= x;
   ELSE
      Z:= y;
   END IF;

   RETURN z;
END; 
BEGIN
   a:= 23;
   b:= 45;

   c := findMax(a, b);
   dbms_output.put_line(' Maximum of (23,45): ' || c);
END;
/

[CREATE  [OR REPLACE ] ] 
FUNCTION function_name [ ( parameter [ , parameter ]... ) ] RETURN 
datatype 
   [ AUTHID { DEFINER  | CURRENT_USER } ] 
   [ PARALLEL_ENABLE 
    [ { [CLUSTER parameter BY (column_name [, column_name ]... ) ] | 
     [ORDER parameter BY (column_name [ , column_name ]... ) ] } ] 
     [ ( PARTITION parameter BY
       { [ {RANGE | HASH } (column_name [, column_name]...)] | ANY } 
) ] 
   ] 
   [DETERMINISTIC]    [ PIPELINED  [ USING implementation_type ] ] 
   [ AGGREGATE  [UPDATE VALUE]  [WITH EXTERNAL CONTEXT] 
USING  implementation_type ]  {IS | AS} 
   [ PRAGMA AUTONOMOUS_TRANSACTION; ] 
   [ local declarations ] 
BEGIN 
   executable statements 
[ EXCEPTION 
   exception handlers ] 
END [ name ]; 

orclz>
orclz> set serverout on
orclz>
orclz> DECLARE
  2     a number;
  3     b number;
  4     c number;
  5  FUNCTION findMax(x IN number, y IN number)
  6  RETURN number
  7  IS
  8      z number;
  9  BEGIN
 10     IF x > y THEN
 11        z:= x;
 12     ELSE
 13        Z:= y;
 14     END IF;
 15
 16     RETURN z;
 17  END;
 18  BEGIN
 19     a:= 23;
 20     b:= 45;
 21
 22     c := findMax(a, b);
 23     dbms_output.put_line(' Maximum of (23,45): ' || c);
 24  END;
 25  /
Maximum of (23,45): 45

PL/SQL procedure successfully completed.

orclz>