Write a query to calculate percentage of total using window functions.

Short answer: Divide each Amount by SUM(Amount) OVER () and multiply by 100. Cast carefully to avoid integer division.

Sample solution

SELECT ProductId, Amount,
       CAST(100.0 * Amount / SUM(Amount) OVER () AS DECIMAL(5,2)) AS PctOfTotal
FROM Sales;

Use 100.0 (not 100) to force decimal math.