Sum of first 50% of rows and remaining 50% of the rows seperately

Question

Hi
I have a table
create table eresults
(student_name varchar2(20),
section_name varchar2(4),
exam_id NUMBER (4))
marks NUMBER (3))
Begin
insert into eresults values ('MOEED', 'A', 1, 20);
insert into eresults values ('SARAH', 'A', 1, 30);
insert into eresults values ('SAM', 'A', 1, 24);
insert into eresults values ('MOEED', 'A', 2, 65);
insert into eresults values ('SARAH', 'A', 2, 100);
insert into eresults values ('SAM', 'A', 2, 4);
insert into eresults values ('MOEED', 'A', 3, 34);
insert into eresults values ('SARAH', 'A', 3, 10);
insert into eresults values ('SAM', 'A', 3, 40);
insert into eresults values ('SARAH', 'A', 4, 33);
insert into eresults values ('SAM', 'A', 4, 99);
end;
/ I want to take a sum of marks, group by student name, for each student in such a way that their first 50% of exams (order by exam id) marks sum is shown seperately in a column and the remaining 50% of the exams summed marks are shown in another column. For students appearing in odd number of exams, like 1,3,5,7 etc - I want the calculation in such a way that first 50% sum will show marks obtained like this: For example a student appeared in 3 exams, so 50% exams shall be 1.5 exams - so the sum of first 50% should be his first exam marks + the 50% of the marks obtained by him in the 2nd or middle or tie breaker exam. And the remaining 50% exam marsk shall be: the 50% of the marks obtain in the 2nd exam + the marks obtained in the 3rd exam. Based on above data, Moeed appeared in 3 exams, and his marks were 20, 65 and 34. So his first 50% marks shall be 20 + 65/2 = 32.5 => 52.5 total marks. And his 2nd 50% marks shall be 65/2 = 32.5 + 34 = 66.5 total marks
I hope I've been able to clear my requiment.
I will appreciate the shortest and simplest possible query to achieve since I've a large data and then I also need to take average of first 50% sum marks divided by 50% of the exams.
Thanks in advance.
regards
Hamza
Edited by: Hamza on May 25, 2011 12:46 AM

Answer 1

TRy this
/* Formatted on 2011/05/24 16:23 (Formatter Plus v4.8.8) */
SELECT student_name, section_name, marks,
       SUM (marks) OVER (PARTITION BY student_name, section_name) sum_marks,
       CASE
          WHEN exam_id = 1
             THEN   marks
                  +   LEAD (marks, 1, 0) OVER (PARTITION BY student_name, section_name ORDER BY exam_id)
                    / 2
          WHEN exam_id = max_id_exam
             THEN   marks
                  +   LAG (marks, 1, 0) OVER (PARTITION BY student_name, section_name ORDER BY exam_id)
                    / 2
          ELSE marks
       END res
  FROM (SELECT student_name, section_name, exam_id, marks,
               MAX (exam_id) OVER (PARTITION BY student_name, section_name)
                                                                  max_id_exam
          FROM eresults)
STUDENT_NAME         SECT      MARKS  SUM_MARKS        RES
MOEED                A            20        119       52.5
MOEED                A            65        119         65
MOEED                A            34        119       66.5
SAM                  A            24        167         26
SAM                  A             4        167          4
SAM                  A            40        167         40
SAM                  A            99        167        119
SARAH                A            30        173         80
SARAH                A           100        173        100
SARAH                A            10        173         10
SARAH                A            33        173         38
11 rows selected.Edited by: Salim Chelabi on 2011-05-24 13:23