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SQL · PRACTICAL GUIDE

SQL Anti Join Patterns

Find records that exist in one set but not another.

Practice SQLLearning path

What this solves

Find expected HUs that were never scanned. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

LEFT JOIN + IS NULL or NOT EXISTS are common anti-join patterns.

SELECT e.HU FROM Expected e
WHERE NOT EXISTS (SELECT 1 FROM Scanned s WHERE s.HU=e.HU);

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

NOT IN can behave unexpectedly when the subquery contains NULL.

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.