# 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.

```sql
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](https://www.ask-codey.com/glossary/subjects/database/master)

Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/unknown-truth-value