Hasan Savran peeks through the windows:
WINDOW Operations in SQL Server can be hard to understand. I believe one of the main reasons for that is the long and repeated code it needs. SELECT…WINDOW Clause will help us to remove repeated code and hopefully it will make the WINDOW Operations more user-friendly.
I am really happy about this syntax change, as often times, we have to re-use the same window frame (PARTITION BY and ORDER BY) or a base frame with a minor change (add one more column to the end). This removes the kind of repetition which makes queries harder to read and introduces subtle bugs.