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SQL · PRACTICAL GUIDE

SQL JOIN Creates Duplicate Rows

Understand why a JOIN returns more rows than expected.

Practice SQLLearning path

What this solves

Diagnose one-to-many matches between PO lines and scans. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

Count matches per join key before joining and verify key uniqueness.

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

DISTINCT can hide the symptom without fixing the relationship that caused it.

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.