NULL handling

The core rule

In SQL, NULL means “unknown,” not “empty string” and not “zero.” That means standard comparisons behave differently.

WHERE col = NULL

This is never true. Use IS NULL or IS NOT NULL instead.

Common patterns

SELECT
  COALESCE(email, 'unknown@example.com') AS email,
  CASE
    WHEN score IS NULL THEN 0
    ELSE score
  END AS score
FROM users;

Why this matters

• filters can silently exclude rows
• aggregate logic can be surprising
• reporting outputs can look inconsistent if nullable values are not normalized

Safety checklist

• decide whether NULL should stay as NULL, or be replaced at the boundary
• use COALESCE for display and fallback logic
• use IS NULL / IS NOT NULL for predicate checks