Unknown Truth Value

The third SQL truth value produced when NULL prevents a condition from being known true or false.

What is Unknown Truth Value in a relational database?

Unknown Truth Value — The third SQL truth value produced when NULL prevents a condition from being known true or false.

SELECT customer_id
FROM customer
WHERE phone_number = NULL;
-- The comparison is UNKNOWN; use IS NULL instead.

Level: Master · More Database terms at this level

Updated September 27, 2026