SUM as a window function

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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 ().