The SQL OVER() clause - when and why is it useful?

Question

    USE AdventureWorks2008R2;
GO
SELECT SalesOrderID, ProductID, OrderQty
    ,SUM(OrderQty) OVER(PARTITION BY SalesOrderID) AS \'Total\'
    ,AVG(OrderQty) OVER(PAR         


        

Answer 1

prkey   whatsthat               cash   
890    "abb                "   32  32
43     "abbz               "   2   34
4      "bttu               "   1   35
45     "gasstuff           "   2   37
545    "gasz               "   5   42
80009  "hoo                "   9   51
2321   "ibm                "   1   52
998    "krk                "   2   54
42     "kx-5010            "   2   56
32     "lto                "   4   60
543    "mp                 "   5   65
465    "multipower         "   2   67
455    "O.N.               "   1   68
7887   "prem               "   7   75
434    "puma               "   3   78
23     "retractble         "   3   81
242    "Trujillo's stuff   "   4   85

That's a result of query. Table used as source is the same exept that it has no last column. This column is a moving sum of third one.

Query:

SELECT prkey,whatsthat,cash,SUM(cash) over (order by whatsthat)
    FROM public.iuk order by whatsthat,prkey
    ;

(table goes as public.iuk)

sql version:  2012

It's a little over dbase(1986) level, I don't know why 25+ years has been needed to finish it up.