1  Select  decode( 'E', '33-39',
  2                  'D', '40-49',
  3             'C', '50-59',
  4             'B', '60-69',
  5             'A', '70-79',
  6             'A+','80% and above'
  7          ),count(tstuid) from test1
  8  group by  decode( 'E', '33-39',
  9                  'D', '40-49',
 10             'C', '50-59',
 11             'B', '60-69',
 12             'A', '70-79',
 13             'A+','80% and above'
 14*         )
SQL> /

DECODE('E','3 COUNT(TSTUID)
------------- -------------
80% and above         28000

 Select  decode(grade, 'E', '33-39',
  2                  'D', '40-49',
  3             'C', '50-59',
  4             'B', '60-69',
  5             'A', '70-79',
  6             'A+','80% and above'
  7          ),count(tstuid) from test1
  8  group by  decode(grade, 'E', '33-39',
  9                  'D', '40-49',
 10             'C', '50-59',
 11             'B', '60-69',
 12             'A', '70-79',
 13             'A+','80% and above'
 14         )
/

function GRADEFormula return Char is
begin
  IF :CF_1>= 80 THEN
  	 RETURN('A+');
  ELSIF :CF_1 >= 70 AND :CF_1 <= 79 THEN
  	RETURN('A');
  ELSIF :CF_1 >= 60 AND :CF_1 <= 69 THEN
  	RETURN('B');
  ELSIF :CF_1 >= 50 AND :CF_1 <= 59 THEN
  	RETURN('C');
  ELSIF :CF_1 >= 40 AND :CF_1 <= 49 THEN
  	RETURN('D');
  ELSIF :CF_1 >= 33 AND :CF_1 <= 39 THEN
  	RETURN('E');
  ELSE 
  	RETURN('F');
  	END IF;
end;

SQL> desc test
 Name                                      Null?    Type
 ----------------------------------------- -------- -------------------
 TID                                       NOT NULL NUMBER(9)
 TDATE                                     NOT NULL DATE
 CLASS                                              VARCHAR2(30)
 EXAMC                                              VARCHAR2(80)
 SUBJECT                                            VARCHAR2(70)
 TOTMARKS                                           NUMBER(3)
 SEC                                                VARCHAR2(50)

SQL> desc test1
 Name                                      Null?    Type
 ----------------------------------------- -------- ------------------------
 TID                                                NUMBER(9)
 TDATE                                              DATE
 TSTUID                                             NUMBER(7)
 OBTMARKS                                           NUMBER(5,2)
 PERCT                                              NUMBER(6,2)
 REMARK                                             VARCHAR2(300)

function CF_1Formula return Number is
begin
  RETURN(Round((NVL(:Sumobtmarks,0)/NVL(:Sumtotmarks,0))*100));
end;

function GRADEFormula return Char is
begin
  IF :CF_1>= 80 THEN
  	 RETURN('A+');
  ELSIF :CF_1 >= 70 AND :CF_1 <= 79 THEN
  	RETURN('A');
  ELSIF :CF_1 >= 60 AND :CF_1 <= 69 THEN
  	RETURN('B');
  ELSIF :CF_1 >= 50 AND :CF_1 <= 59 THEN
  	RETURN('C');
  ELSIF :CF_1 >= 40 AND :CF_1 <= 49 THEN
  	RETURN('D');
  ELSIF :CF_1 >= 33 AND :CF_1 <= 39 THEN
  	RETURN('E');
  ELSE 
  	RETURN('F');
  	END IF;
end;

SELECT grade, count(*)
FROM (select student_id, <calculation of grade>
      from .....
      where .....
     )
group by grade

select distinct :grade,count(*) from 
(select stuid,:grade,examc,test.class,sec
 from student,dual,test,test1
where student.status='PRESENT'
and test.class=:cls --(pick through Parameter Form)
and test.sec=:sess  --(pick through Parameter Form)
and test.examc=:ty  --(pick through Parameter Form)
group by :grade,stuid,examc,test.class,sec)

SELECT grade, count(*)
FROM (select student_id, 
             CASE WHEN percentage >= 80 THEN 'A+'
                  WHEN percentage >= 70 THEN 'A'
                  .....
             END AS grade        
      from (SELECT student_id, SUM(obtmarks)/SUM(totmarks) AS percentage
            FROM ....
            WHERE .....
            GROUP BY student_id
           )
     )
group by grade

 SELECT grade, count(*)
 FROM (select stuid,
              CASE WHEN perc >= 80 THEN 'A+'
                   WHEN perc between 70 and 79 THEN 'A'
                   WHEN perc between 60 and 69 THEN 'B'
                   WHEN perc between 50 and 59 THEN 'C'
                   WHEN perc between 40 and 49 THEN 'D'
                   WHEN perc between 33 and 39 THEN 'E'
                   WHEN perc < 33  THEN 'F'
              END AS grade
       from (SELECT stuId, SUM(obtmarks)/SUM(totmarks)*100 AS perc,EXAMC,SEC,TEST.CLASS
             FROM TEST,TEST1,STUDENT,CLASS,SUB
         where test.tid=test1.tid
        and test.tdate=test1.tdate
        and test.class=:cls
        and test.sec=:ssec
        and test1.tstuid=student.stuid
        and student.STATUS='PRESENT'
        and sub.subject=test.subject
        and sub.cname=class.cname
        and examc=:TY
        group by TEST.class,examc,sec,stuid)
             GROUP BY stuid,perc
            )
 group by grade
Out put is:
SQL> /

GR   COUNT(*)
-- ----------
A           1
A+          1
B           5
C           8
D           2
E           3
F           2
            3

SELECT grade, count(*)
 FROM (select stuid,
              CASE WHEN perc >= 80 THEN 'A+'
                   WHEN perc between 70 and 79 THEN 'A'
                   WHEN perc between 60 and 69 THEN 'B'
                   WHEN perc between 50 and 59 THEN 'C'
                   WHEN perc between 40 and 49 THEN 'D'
                   WHEN perc between 33 and 39 THEN 'E'
                   WHEN perc < 33  THEN 'F'
              END AS grade
       from (SELECT stuId, SUM(obtmarks)/SUM(totmarks)*100 AS perc,EXAMC,SEC,TEST.CLASS
             FROM TEST,TEST1,STUDENT,CLASS,SUB
         where test.tid=test1.tid
        and test.tdate=test1.tdate
        and test.class=:cls
        and test.sec=:ssec
        and test1.tstuid=student.stuid
        and student.STATUS='PRESENT'
        and sub.subject=test.subject
        and sub.cname=class.cname
        and examc=:TY
        group by TEST.class,examc,sec,stuid)
             GROUP BY stuid,perc
            )
 group by grade