Mysql best students in every class in a school - mysql

Question

In MySql I need to select top student in every class in a school in termid=10 to get discount for next term enrollment .
Please notice that total is not in table(I put in below for clearing problem)
I have this workbook table for all students workbook:
id studentid classid exam1 exam2 total termid
1 2 11 20 40 60 10
2 1 22 40 20 60 10
3 4 11 40 20 60 10
4 5 33 10 60 70 10
5 7 22 10 40 50 10
6 8 11 10 30 40 10
7 9 33 20 45 65 10
8 11 11 null null null 10
9 12 54 null null null 02
10 13 58 null null null 02
1st challenge is : exam1 and exam2 are VARCHAR and total is not in table (as i explained).
2nd challenge is : as you can see in id=8 std #11 has not numbers
3rd challenge is : may be two students have top level so they must be in result.
I need result as :
id studentid classid exam1 exam2 total termid
1 2 11 20 40 60 10
3 4 11 40 20 60 10
4 5 33 10 60 70 10
2 1 22 40 20 60 10
i have this query but not work good as i mention.
SELECT DISTINCT id,studentid,classid,exam1,exam2,total,termid ,(CAST(exam1 AS DECIMAL(9,2))+CAST(exam2 AS DECIMAL(9,2))) FROM workbook WHERE ClassId = '10';

Answer 1

You can get the total for the students by just adding the values (MySQL will convert the values to numbers). The following gets the max total for each class:
select w.classid, max(coalesce(w.exam1, 0) + coalesce(w.exam2, 0)) as maxtotal
from workbook w
group by w.classid;
You can then join this back to the original data to get information about the best students:
select w.*, coalesce(w.exam1, 0) + coalesce(w.exam2, 0) as total
from workbook w join
(select w.classid, max(coalesce(w.exam1, 0) + coalesce(w.exam2, 0)) as maxtotal
from workbook w
group by w.classid
) ww
on w.classid = ww.classid and (coalesce(w.exam1, 0) + coalesce(w.exam2, 0)) = ww.maxtotal;

Answer 2

Another approach is to join the table with itself. You find out the max for each class and then join all students of this class which match the class max:
max for each class (included in the final statement already):
SELECT classid, MAX(CAST(exam1 AS UNSIGNED) + CAST(exam2 AS UNSIGNED)) as 'maxtotal'
FROM students
WHERE NOT ISNULL(exam1)
AND NOT ISNULL(exam2)
GROUP BY classid
The complete statement:
SELECT s2.*, s1.maxtotal
FROM (SELECT classid, MAX(CAST(exam1 AS UNSIGNED) + CAST(exam2 AS UNSIGNED)) as 'maxtotal'
FROM students
WHERE NOT ISNULL(exam1)
AND NOT ISNULL(exam2)
GROUP BY classid) s1
JOIN students s2 ON s1.classid = s2.classid
WHERE s1.maxtotal = (CAST(s2.exam1 AS UNSIGNED) + CAST(s2.exam2 AS UNSIGNED));
SQL Fiddle: http://sqlfiddle.com/#!2/9f117/1

Answer 3

Use a simple Group by Statement:
SELECT
studentid,
classid,
max(coalesce(exam1,0)) as max_exam_1,
max(coalesce(exam2,0)) as max_exam_2,
sum(coalesce(exam1,0) + coalesce(exam2,0)) as sum_exam_total,
termid
FROM
workbook
WHERE
termid=10
GROUP BY
1,2
ORDER BY
5

Answer 4

Try something like this:
SELECT id,studentid,classid,exam1,exam2,(CAST(exam1 AS DECIMAL(9,2))+CAST(exam2 AS DECIMAL(9,2))) AS total,termid FROM `workbook` WHERE ((CAST(exam1 AS DECIMAL(9,2))+CAST(exam2 AS DECIMAL(9,2)))) > 50

Answer 5

Thanks all my friends
I think combine between 2 answer in above is best :
SELECT s2.*, s1.maxtotal
FROM (SELECT ClassId, MAX(
coalesce(exam1,0)+
coalesce(exam2,0)
) as 'maxtotal'
FROM workbook
WHERE
(
termid = '11'
)
GROUP BY ClassId) s1
JOIN workbook s2 ON s1.ClassId = s2.ClassId
WHERE s1.maxtotal = (
coalesce(exam1,0)+
coalesce(exam2,0)
) AND (s1.maxtotal >'75');
last line is good for s1.maxtotal=0 (some times student scores have not be entered and all equals 0 so all will shown as best students) or some times we need minimum score (to enroll in next term).
So thanks all