Have you ever had a sub-select where you really needed to reference a value in the outer query? I know I have! The naive way would be to run the outer query and then loop over the results running the inner query on each one. Luckily, there’s a better way. The Correlated subquery. Check it out! The example given is

SELECT employee_number, name
  FROM employees AS Bob
  WHERE salary > (
    SELECT AVG(salary)
      FROM employees
      WHERE department = Bob.department);

See how the sub-select references the outer query? It’s SQL magic.