SUM with an empty OVER clause: how many rows collapse?

Question

Accepted Answer

SUM(amount) OVER () with an empty OVER clause aggregates over the entire result set. grand_total = 100+200+150 = 450 for every row. The key difference from GROUP BY: the original rows are preserved - each row keeps its individual amount AND gets the total appended. This makes it easy to compute each row's percentage of the total: amount / SUM(amount) OVER ().