Explanation/Reference:
Not that employees is used both in the first line (UPDATE employees) and later (FROM employees, departments). This would not cause the correct output. Instead aliases should be use.
The following would be the correct query:
UPDATE employees a
SET department_id
(SELECT department_id
FROM departments
WHERE location_id = '2100'),
(salary, commission_pct)
(SELECT 1.1*AVG(salary), 1.5*AVG(commission_pct)
FROM employees b
WHERE a.department_id = b.department_id)
WHERE department_id IN
(SELECT department_id
FROM departments
WHERE location_id = 2900
OR location_id = 2700);
Reference: http://docs.oracle.com/database/121/SQLRF/statements_10008.htm#SQLRF01708