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SQL · PRACTICAL GUIDE

SQL Find Duplicate Values

Identify keys that occur more often than expected.

Practice SQLLearning path

What this solves

Find duplicate HU numbers or duplicate invoice lines. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

GROUP BY the business key and keep counts greater than one.

SELECT HU, COUNT(*) AS Rows
FROM Scans
GROUP BY HU
HAVING COUNT(*)>1;

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.

Common mistake

Define the true business key first; duplicate full rows and duplicate keys are different problems.

Try it next

PracticeChange the example and verify the result.Use a toolMove from a small example to real work.Related contentFind guides, errors and scenarios on this topic.