The SUM function calculates the total of the values specified by
    the value expression in its argument. If there are no rows in the
    result table to which the SUM function is applied, it returns a
    null value.

    The SUM function must refer to a value with a numeric or INTERVAL
    data type. It returns a value of the same general data type
    (fixed- or floating-point) big enough to store the result.

    If your dialect is set to an ANSI/ISO SQL standard, a warning
    message is returned if any of the values is NULL.