null values to be replaced by a default value

Hi,

To return one value not null, i've created a function :

CREATE FUNCTION isnull (int4,int4 ) RETURNS int4 AS '
BEGIN
IF ($1 IS NULL) THEN
RETURN $2;
ELSE
RETURN $1;
END IF;
END;
' LANGUAGE 'plpgsql';

select isnull(my_int,0) as number, sample_id from my_table;

but I got
number |sample_id
------+---------
| 13
| 15

What should I do to have '0' in the column 'number' instead of NULL ?
Thanks,

nuch