Group by based upon condition

Question

Database : SQL Server 2000 Enterprise Edition
OS : Windows Server 2003
Huh?  Every one will ask me why I am posting SQL Server question in Oracle forum ?  I shall reply something like this :
Actually my friend is SQL Server DBA and he is getting problem to solve the below query.  He asked me to solve in Oracle 9i (Because since he is using SQL Server 2000, so I guess it is almost similar level) and/or through pure SQL; but I am really very poor in SQL (need to read docs more times); I failed to help him.  He just asked me how to solve in Oracle and if it is solve in Oracle then he will try to solve in by looking similar function in SQL Server's functions etc.  Even though, I have posted the question in two SQL Server forums, but probably there is less activity, so I am posting here :
create table student
rollno int,
name varchar(30),
class varchar(20)
insert into student values (1,'Robert','IT');
insert into student values (2,'John','Arts');
insert into student values (2,'Hussain','IT');
insert into student values (1,'David','Science');
insert into student values (3,'Polo','IT');
insert into student values (2,'Jonathan','Science');
insert into student values (4,'Joseph','History');
insert into student values (1,'Richard','History');
insert into student values (1,'Michel','Commerce');
insert into student values (1,'Albert','Geography');
SQL> select * from student;
    ROLLNO NAME                           CLASS
         1 Robert                         IT
         2 John                           Arts
         2 Hussain                        IT
         1 David                          Science
         3 Polo                           IT
         2 Jonathan                       Science
         4 Joseph                         History
         1 Richard                        History
         1 Michel                         Commerce
         1 Albert                         Geography
10 rows selected.
SQL>
Required Output :
rollno  name        class
1        Robert      IT
2        Hussain     IT
3        Polo        IT
4        Joseph      History
Logic behind required ouput : Rollno should be unique from all the classes giving priority to :
1.IT
2.Science
3.Arts
4.Commerce
5.Geography
6.Anyone
means, if suppose we are going to fetch a unique roll number (x), and if it is exists in all the classes then first it should be from IT, and if it is not in IT class then it should search in Science Class, if it is not in IT and Science class, then it should search in Arts class, if it is not in IT, Science,Arts then it should search in Commerce Class; like wise. All existing roll numbers must be unique from all classes giving priority to IT, Science, Arts, Commerce, Geography and if that unique roll number is not from above 5 classes, then it may be of any one class.
Kindly let me know, if I am unclear in my question and/or needs to provide more details. Since my friend is using SQL Server 2000, so I would like to request to please help me/him by using simple SQL please.
Thanks and Regards
Girish Sharma

Answer 1

Probably need more test cases:
        SELECT ROLLNO, NAME, CLASS
  FROM (SELECT a.*,
               RANK ()
               OVER (
                  PARTITION BY rollno
                  ORDER BY
                     (CASE
                         WHEN class = 'IT' THEN 1
                         WHEN class = 'Science' THEN 2
                         WHEN class = 'Arts' THEN 3
                         WHEN class = 'Commerce' THEN 4
                         WHEN class = 'Geography' THEN 5
                         ELSE 6
                      END))
                  rnk
          FROM student a)
WHERE rnk = 1;
ROLLNO NAME CLASS
1 Robert IT
2 Hussain IT
3 Polo IT
4 Joseph History
Cheers,
Manik.