How to find top 3 topper of each subject in given table - mysql

Question

id - Name - Subject - Marks
1 - ABC - MAT - 90
2 - ABC - SCI - 80
3 - ABC - ENG - 90
4 - ABC - HIS - 96
5 - ABC - PHY - 70
6 - ABC - CHE - 43
7 - XYZ - MAT - 90
8 - XYZ - SCI - 80
9 - XYZ - ENG - 90
10 - XYZ - HIS - 96
11 - XYZ - PHY - 70
13 - XYZ - CHE - 43
etc .....
Just want to show 3 topper of each subject
ABC - MATH - 90
XYZ - MATH - 90
DEF - MATH - 80
etc

Answer 1

You can do this using variables.
select t.*
from (select t.*,
(#rn := if(#s = subject, #rn + 1,
if(#s := subject, 1, 1)
)
) as rn
from t cross join
(select #rn := 0, #s := '') params
order by subject, marks desc
) t
where rn <= 3
order by t.subject, t.rn;

Answer 2

With your data I create a SqlFiddleDemo.
I include two addtional student so query return only 3 from 4.
CREATE TABLE Courses
(`id` int, `Name` varchar(3), `Subject` varchar(3), `Marks` int);
And create this derivated to simplify the next code. The idea is create an unique code with 3 number 000-100 + Name so I can sorted during left outer join using AND L.comb <= R.comb.
Take note because this sorting in case of tie the latest name alphabetic will be showing first.
CREATE TABLE s_course AS
SELECT `id`, `Name`, `Subject`, `Marks`, concat(LPAD(`Marks`, 3, '0'), `Name`) as comb
FROM Courses;
Now the SELECT, if you run the inner select will see the 4 result and check how the ties are resolve.
SELECT *
FROM (
SELECT L.Subject, L.Marks, L.Name, count(*) as rn
FROM s_course L
left outer join s_course R
ON L.Subject = R.Subject
AND L.comb <= R.comb
GROUP BY L.Subject, L.comb
ORDER BY L.Subject, L.comb
) t
WHERE rn <= 3
ORDER BY Subject, rn
OR maybe you can ORDER BY Subject, Marks DESC, Name
This query exploit one issue of MySQL where you dont need put the same fields on select and group by
OUTPUT
| Subject | Marks | Name | rn |
|---------|-------|------|----|
| CHE | 48 | PQR | 1 |
| CHE | 48 | FGH | 2 |
| CHE | 43 | XYZ | 3 |
|---------|-------|------|----|
| ENG | 95 | PQR | 1 |
| ENG | 92 | FGH | 2 |
| ENG | 90 | XYZ | 3 |
|---------|-------|------|----|
| HIS | 96 | XYZ | 1 |
| HIS | 96 | ACB | 2 |
| HIS | 91 | PQR | 3 |
|---------|-------|------|----|
| MAT | 95 | PQR | 1 |
| MAT | 95 | FGH | 2 |
| MAT | 90 | XYZ | 3 |
|---------|-------|------|----|
| PHY | 75 | PQR | 1 |
| PHY | 70 | XYZ | 2 |
| PHY | 70 | ACB | 3 |
|---------|-------|------|----|
| SCI | 80 | XYZ | 1 |
| SCI | 80 | ACB | 2 |
| SCI | 75 | PQR | 3 |

Answer 3

select m1.id, max(m1.marks) 'marks', max(m1.subject) 'subject'
from marks m1
left join marks m2 on m2.subject =m1.subject and ( m2.marks>=m1.marks )
left join marks m3 on (m3.subject =m2.subject) and m3.id<>m1.id and m3.id<>m2.id and m3.marks>=m2.marks
left join marks m4 on (m4.subject =m3.subject) and m4.id<>m1.Id and m4.id<>m2.id and m4.id <>m3.id and m4.marks>=m3.marks
left join marks m12 on (m12.subject =m1.subject and m12.subject =m2.subject ) and m12.marks>m1.marks and m12.marks<m2.marks
left join marks m23 on (m23.subject=m2.subject and m23.subject=m3.subject) and ( m23.marks>m2.marks and m23.marks < m3.marks)
where
(m4.id is null)
and m12.id is null
and m23.id is null
group by m1.id
order by max(m1.subject) asc, max(m1.marks) desc