title: Can I group by in SQL query with window function?
body: <p>I need to get <strong>employees with smallest salary in their departments</strong>
  I did it using anti join.</p>
  
  <pre><code>     select emp.employee_id,emp.last_name,emp.salary,emp.department_id
       from employees emp
       left join employees sml 
       on sml.department_id = emp.department_id and sml.salary &lt; emp.salary
       where sml.employee_id is null and emp.department_id is not null
  </code></pre>
  
  <p>But I've been told that it's possible to do it using window function <strong>using one select</strong>.
  However I can't group it by department_id and use it at the same time.
  Is that a bug or me being stupid?</p>
  
  <pre><code>     SELECT  department_id,
       min(salary) OVER (partition by department_id)  as minsalary
       FROM employees;
       GROUP BY department_id
  </code></pre>
  
  <p>SQL Developer says 00979. 00000 -  "not a GROUP BY expression"</p>