Query to Get Grade History using Lag & Lead Function

SELECT CASE

          WHEN effective_start_date <>

                  LAG (effective_end_date, 1)

                     OVER (ORDER BY effective_end_date)

                  + 1

          THEN

             LAG (effective_end_date, 1) OVER (ORDER BY effective_end_date)

             + 1

          ELSE

             effective_start_date

       END

          date_from,

       effective_end_date date_to,

       (SELECT name

          FROM per_grades pg

         WHERE grade_id = prev)

          grade

  FROM (  SELECT effective_start_date,

                 effective_end_date,

                 grade_id prev,

                 LEAD (grade_id, 1, 0) OVER (ORDER BY effective_start_date) nex

            FROM per_all_assignments_f paaf

           WHERE assignment_number = '881'

        ORDER BY effective_start_date)

 WHERE prev <> nex