NULL

NULL represents a missing or unknown value — not zero and not an empty string. Crucially, NULL is never equal to anything, not even another NULL.

Because NULL = NULL is unknown (not true), you must test for it with IS NULL / IS NOT NULL, never = NULL. Joins on a NULL key never match, and aggregates like AVG silently skip NULLs.

Functions such as COALESCE, NVL, and IFNULL substitute a default when a value is NULL, which is essential for correct arithmetic and comparisons.

Example

SELECT first_name, NVL(commission_pct, 0) AS commission
FROM   employees
WHERE  commission_pct IS NULL;