CAML中orderby的使用

<Where>
    <Or>
      <IsNull>
        <FieldRef Name="Expires" />
      </IsNull>
      <Geq>
        <FieldRef Name="Expires" />
        <Value Type="DateTime">
          <Today />
        </Value>
      </Geq>
    </Or>
   </Where>
   <OrderBy>
   <FieldRef Name="Modified" Ascending="FALSE" />
  </OrderBy>