def format_non_null_ratio(rows, nulls):
    """Format percentage of non-null value of the column

    Args:
        rows (int): number of rows
        nulls (int): number of null-values

    Returns:
        str: non-null ratio with the format '%.2f'.
    """
    if rows is None or nulls is None:
        return 'N/A'

    if rows == 0:   # avoid devide by zero error.
        return "%.2f %%" % 0
    return "%.2f %%" % (float(rows - nulls) / rows * 100.0)