imes current data is not needed.
    Consider a table which records sales:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE invoice (
    invoice_no    integer        PRIMARY KEY,
    seller_no     integer,       -- ID of salesperson
    invoice_date  date,          -- date of sale
    invoice_amt   numeric(13,2)  -- amount of sale
);</PRE
><P>

    If people want to be able to quickly graph historical sales data, they
    might want to summarize, and they may not care about the incomplete data
    for the cu