SQL GROUP BY Multiple Columns

What this solves

Count handling units by PO and material. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

Every non-aggregated selected column belongs in the grouping grain.

SELECT PO, Material, COUNT(*) HU
FROM Scans
GROUP BY PO, Material;

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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