Answer
NULL is a marker for a missing or unknown SQL value. • Most comparisons involving NULL produce the unknown truth value. • NULL is not equal to zero, an empty string, or another NULL through ordinary equality. • Functions and aggregates have documented null-handling rules.
💡 SQL Example
SELECT COALESCE(nickname, name) FROM customers;
Result
A non-null display name
⚡ Quick Revision
NULL marks missing or unknown information and needs null-aware operations.