MEPX
Chapter 3 of 10All chapters
  1. 1What SQL is
  2. 2SELECT and WHERE
  3. 3NULL
  4. 4Sorting and limiting
  5. 5Aggregates and GROUP BY
  6. 6Joins
  7. 7Keys and relationships
  8. 8Subqueries and CTEs
  9. 9Indexes and performance
  10. 10Changing data

Chapter 3 of 10

NULL

Unknown, not empty.

Why it is different

NULL means the value is unknown. Comparing anything with NULL gives unknown rather than true, so column = NULL never matches, even for rows that are NULL.

  • Use IS NULL and IS NOT NULL instead.
  • COALESCE(column, fallback) substitutes a value when one is missing.

Where it bites

Aggregates skip NULLs, so AVG ignores them rather than treating them as zero. A NOT IN list containing a NULL returns nothing at all, which is the classic silent bug.