SQL LEFT JOIN to Find Missing Rows
Keep every source row and expose records with no match.
What this solves
Find POs with no matching inventory master record. This pattern is useful when the business question is clear but a generic tutorial is too abstract.
The rule
LEFT JOIN then test the right-side key for NULL.
SELECT o.PO
FROM Orders o
LEFT JOIN Inventory i ON i.Material=o.Material
WHERE i.Material IS NULL;Real-work checklist
- Define the business key and expected grain before writing the formula, query or markup.
- Test the pattern on a small known sample where you can verify the answer manually.
- Check missing values, duplicates and data types before trusting a large result.
- Scale to the real file or database only after the logic is proven.
Common mistake
Filtering the right table in WHERE can accidentally turn a LEFT JOIN into an INNER JOIN.