Compute the total of SUM(column)
This is valid syntax, too: 

       sum(sum(qty)) over ()


It's a bit confusing when one sees it at first but you only have to remember that the window functions - e.g. sum() over () - are applied after the group by so everything that can appear in the select list of a group by query can be placed inside a window aggregate. So (the qty cannot but) the sum(qty) can be placed inside sum() over ():

select sum(qty), itemid, proddte, 
       sum(sum(qty)) over () as grandtotal  
from testtable 
where .... 
group by itemid, proddte ;




Having said that, I'd prefer the GROUPING SETS query provided by Aaron Bertrand. The total sum needs to be shown once and not in every row. 

Also note that while the sum of sums can be used to calculate the total sum, if you wanted the total count, you'd have to use the sum of counts (and not the count of counts!):

sum(count(*)) over ()  as grand_count


And if one wanted the average over all the table, it would be even more complicated:

sum(sum(qty)) over ()
/ sum(count(qty)) over ()  as grand_average


because the average of averages is not the same as the average over all. (If you try the avg(avg(qty)) over () you'll see that it may yield a different result than the above grand average.)