SQL Top N Per Group

What this solves

Find the two most expensive quotes per material. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

Use ROW_NUMBER or DENSE_RANK partitioned by the group.

WITH q AS (SELECT *, ROW_NUMBER() OVER(PARTITION BY Material ORDER BY Price DESC) rn FROM Quotes)
SELECT * FROM q WHERE rn<=2;

Real-work checklist

1. Define the business key and expected grain before writing the formula, query or markup.
2. Test the pattern on a small known sample where you can verify the answer manually.
3. Check missing values, duplicates and data types before trusting a large result.
4. Scale to the real file or database only after the logic is proven.

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