Create a function with optional arguments in MySQL

You cannot set optional parameters in MySQL stored procedures.
You can however set optional parameters in a MySQL UDF.

You do know that MySQL has an AVG aggregate function?

Workaround If you can face the ugliness of this workaround here's samplecode that uses a comma separated string with values as input and returns the average.

DELIMITER $$

CREATE FUNCTION MyAvg(valuestr varchar) RETURNS float
BEGIN
  DECLARE output float;
  DECLARE arg_count integer;
  DECLARE str_length integer;
  DECLARE arg float;
  DECLARE i integer;

  SET output = NULL;

  SET i = LENGTH(valuestr);
  IF i > 0 THEN BEGIN 

    SET arg_count = 1;
    WHILE i > 0 DO BEGIN
      IF MID(valuestr, i, 1)
      SET i = i - 1;
    END; END WHILE;

    /* calculate average */
    SET output = 0;
    SET i = arg_count;
    WHILE i > 0 DO BEGIN
      SET arg = SUBSTRING_INDEX( 
                  SUBSTRING_INDEX(valuestr, ',' , i)
                  , ',', -1 );
      SET output = output + arg;
      SET i = i - 1; 
    END; END WHILE;       
    SET output = output / arg_count;

  END; END IF;    
  RETURN output;
END $$

DELIMITER ;

Use concat_ws to feed the function.

SELECT MyAvg(CONCAT_WS(',',100,200,300,500)) AS test;

You can also write an UDF in C(++) or Delphi/Lazarus

While far from an ideal solution, here's how I solved optional parameters for a concat function I needed:

delimiter ||
create function safeConcat2(arg1 longtext, arg2 varchar(1023))
 returns longtext
 return safeConcat3(arg1, arg2, '');
||

create function safeConcat3(arg1 longtext, arg2 varchar(1023), arg3 varchar(1023))
 returns longtext
 return safeConcat4(arg1, arg2, arg3, '');
||

create function safeConcat4(arg1 longtext, arg2 varchar(1023), arg3 varchar(1023), arg4 varchar(1023))
returns longtext
  begin
      declare result longText;
      set result = concat(arg1, arg2, arg3, arg4);
      if( result is null) then
          set result=arg1;
      end if;
      return result;
  end
||

Note: This means you have to call the method that corresponds to the number of args.

Another approach is to pass only one 'super' parameter which is string with commas in it separating the real parameters. The mysql procedure can then parse the 'super' parameter into the separate real parameters. Example:

create procedure procWithOneSuperParam(param1 varchar(500))
declare param2 varchar(100);
begin
 if LOCATE(',',param1) > 0 then
  .. param2=<extract the string after the ',' from param1> ..