# Foreign Key

Columns whose values must match a referenced unique or primary key, subject to NULL rules.

## What is **Foreign Key** in a relational database?

**Foreign Key** — Columns whose values must match a referenced unique or primary key, subject to NULL rules.

```sql
CREATE TABLE purchase_order (
    order_id INTEGER PRIMARY KEY,
    customer_id INTEGER REFERENCES customer (customer_id)
);
```

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

Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/foreign-key