How does NULL work in SQL and what are its implications?

NULL represents missing or unknown data - it's not zero, empty string, or false. NULL comparisons require IS NULL or IS NOT NULL (using = returns UNKNOWN, not TRUE/FALSE). NULL in calculations propagates: 5 + NULL = NULL. Aggregate functions ignore NULLs (except COUNT(*)). NVL/COALESCE/IFNULL handle NULLs. NULL affects index usage, join behavior, and sorting. Design carefully: sometimes a default value is better than allowing NULLs.