Correlated Subquery in ORDER BY

Question

The following is a valid sql. But, what is meant by a correlated scalar subquery in the ORDER BY clause?
SELECT employee_id, last_name
FROM employees e
ORDER BY (SELECT department_name
FROM departments d
WHERE e.department_id=d.department_id);

Answer 1

It is exactly the same in the ORDER BY clause as in any other part of a query. As biswabijay says, the correlated query is run once for each row in the result set.
In essence, the fileds selected from the correlated query are added to the result for each row, then evaluated in the appropriate context (e.g. compared in a WHERE clause, used for sorting in an ORDER BY).
A small addition to your original query makes this more apparent.
SQL> SELECT employee_id, last_name, department_name
  2  FROM employees e, departments d
  3  WHERE e.department_id=d.department_id
  4  ORDER BY (SELECT department_name
  5  FROM departments d
  6  WHERE e.department_id=d.department_id);
EMPLOYEE_ID LAST_NAME                 DEPARTMENT_NAME
        205 Higgins                   Accounting
        206 Gietz                     Accounting
        200 Whalen                    Administration
        100 King                      Executive
        101 Kochhar                   Executive
        102 De Haan                   Executive
        185 Bull                      Shipping
        144 Vargas                    Shipping
        143 Matos                     Shipping
        184 Sarchand                  ShippingHTH
John