# NULL

A marker indicating that a value is missing or unknown, rather than zero or an empty string.

## What is **NULL** in a relational database?

**NULL** — A marker indicating that a value is missing or unknown, rather than zero or an empty string.

```sql
SELECT customer_id
FROM customer
WHERE phone_number IS NULL;
```

Level: Novice · [More Database terms at this level](https://www.ask-codey.com/glossary/subjects/database/novice)

Updated: September 27, 2026

## In other subjects

- [null in JavaScript](https://www.ask-codey.com/glossary/null)

Source: https://www.ask-codey.com/glossary/null-database