In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. Because the subquery may be evaluated once for each row processed by the outer query, it can be slow. Here is an example for a typical correlated subquery. In this example, the objective is to find all employees whose salary is above average for their department. SELECT employee_number, name FROM employees emp WHERE salary > ( SELECT AVG(salary) FROM employees WHERE department = emp.department);