Why NULL and three-valued logic cause errors

SQL predicates can produce TRUE, FALSE, or UNKNOWN. NULL represents missing or unknown information, so status = NULL does not find missing values, and NOT does not automatically turn UNKNOWN into TRUE.

Use IS NULL or IS NOT NULL for filtering and COALESCE for display fallbacks. COUNT(*), COUNT(column), SUM, and AVG also treat NULL differently, so define the intended denominator and missing-value meaning before aggregating.

Do not replace every NULL with zero or an empty string. An unknown amount, a zero amount, and an unfilled text field often represent different business states.