# Correlated Subquery

A subquery that refers to values from an outer query and is evaluated in that context.

## What is **Correlated Subquery** in a relational database?

**Correlated Subquery** — A subquery that refers to values from an outer query and is evaluated in that context.

```sql
SELECT customer_id
FROM customer
WHERE EXISTS (
    SELECT 1
    FROM purchase_order
    WHERE purchase_order.customer_id = customer.customer_id
);
```

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Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/correlated-subquery