Oracle function： row_number()

1.
      select *
        from (select *
                from (select t.*, row_number() OVER(ORDER BY null) AS "row_number"
                        from twitter_accounts t) p
               where p."row_number" > 0 and p."row_number" <=2) q
       where rownum <= 10
2.

SELECT department_id, last_name, employee_id, ROW_NUMBER()
   OVER (PARTITION BY department_id ORDER BY employee_id) AS emp_id
   FROM employees;

3. 

SELECT last_name FROM 
   (SELECT last_name, ROW_NUMBER() over (order by last_name) R FROM employees)
   WHERE R BETWEEN 51 and 100;