In a SQL database query, a correlated sub-query (also known as a synchronized subquery) is a sub-query (a query nested inside another query) that uses values from the outer query in its WHERE clause. The sub-query is evaluated once for each row processed by the outer query.
Here is an example for a typical correlated sub-query. In this example we are finding the list of employees (employee number and names) having more salary than the average salary of all employees in that employee's department
SELECT employee_number, name
FROM employee AS e1
WHERE salary > (SELECT avg(salary)
WHERE department = e1.department);