Partition By Null

I saw 'Partition by Null' in a snippet of code on the web, but with no explanation. Well, I'm so glad I tried this out, because as shown in the code below, this will give a calculation on a row basis.

With out the 'Partition By' , attempting to sum by row would display 'not a single-group group function.' 'Over (Partition By Null)' can also be written as simply 'Over()'.

Break On Report
Compute Sum Of        -
               Sal    -
               PctSal    On Report

Column Pctsal Heading '% Of Total|Salary'  Format 999

Select Ename,
       Sal,
       -----------------------------------------
             100*Sal/Sum(Sal)
                     Over (Partition By Null)
             As
       Pctsal
       -----------------------------------------
  From Emp
 Order By Sal Desc;

See Oracle documentation for further detail:
Partition By