EXISTS Subqueries

EXISTS returns TRUE if the subquery returns any rows:

SELECT * FROM customers c
WHERE EXISTS (
    SELECT 1 FROM orders o
    WHERE o.customerId = c.id
);

This finds customers who have at least one order.

EXISTS vs IN

• EXISTS is often faster for large datasets
• EXISTS stops searching once it finds one match
• Use EXISTS when you just need to know "does it exist?"

Your Task

Find all customers who have at least one completed order (status = 'completed').

Return customer name and email.