What is a subquery?

SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

What type of subquery is this?

SELECT name, salary, (SELECT AVG(salary) FROM employees) AS avg_salary FROM employees;

Where is the subquery placed?

SELECT * FROM employees WHERE dept_id IN (SELECT id FROM departments WHERE location = 'New York');

What operator is used with this subquery?

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

What does EXISTS check?