f rows defined by this virtual table.
   </p><p>
    We already saw that <code class="literal">ORDER BY</code> can be omitted if the ordering
    of rows is not important.  It is also possible to omit <code class="literal">PARTITION
    BY</code>, in which case there is a single partition containing all rows.
   </p><p>
    There is another important concept associated with window functions:
    for each row, there is a set of rows within its partition called its
    <em class="firstterm">window frame</em>.