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SQL · PRACTICAL GUIDE

SQL NULL Handling

Work safely with missing or unknown values.

Practice SQLLearning path

What this solves

Show missing supplier lead times without breaking arithmetic. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

Use IS NULL for tests and COALESCE for defaults.

SELECT Supplier, COALESCE(LeadTimeDays,0) AS LeadTimeDays
FROM Suppliers;

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

NULL = NULL is not true in SQL; use IS 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.