Find the total score. for each student - mysql-5.7

Question

Hi I have a query to retrive students score in this format
Id subject class_score total_marks rank subject_type
1 MATLAB 33.80 73.30 11 Core
1 SCIENCE 39.50 83.00 4 Core
1 ENGLISH 37.60 77.60 8 Elective
1 WQP 43.90 77.40 9 Core
1 BDT 42.00 71.50 12 Elective
1 ART 47.10 84.60 2 Elective
1 COMPUTING 26.00 65.50 13 Elective
2 MATLAB 0.00 0.00 14 Core
2 SCIENCE 38.60 73.60 10 Core
2 ENGLISH 43.80 83.30 3 Elective
2 WQP 45.00 89.00 1 Core
2 BDT 41.00 79.50 6 Elective
2 ART 38.00 78.00 7 Elective
2 COMPUTING 40.80 80.80 5 Elective
I wish to calculate the overall score of each student by add the total_marks of all (Core subject + the top 3 score of the student's elective subjects) and rank them from first to last.Please can anyone assist ?Thanks
My current
use ods_sms;
SELECT student_id,sum(t.total_marks)score
FROM
(select
student_id,total_marks from tab_exam_tracker
where (subject_type='Core')
union all select t.student_id,t.total_marks from
(select student_id,total_marks from tab_exam_tracker
where subject_type = 'Elective' order by total_marks desc
limit 3) t
) t
GROUP BY student_id
But am getting wrong results

Answer 1

SELECT
t.id,
SUM(total_marks) AS Core_Marks,
Elective_Marks
FROM tab_exam_tracker t
INNER JOIN (
SELECT id, SUM(total_marks) as Elective_Marks
FROM (
SELECT
id,
total_marks,
IF(#id <> id, #rank:= 1, #rank:= #rank + 1) as rank,
#id:= id
FROM tab_exam_tracker,
(SELECT #id:= 0 r_id, #rank:= 0 AS r) AS a
WHERE subject_type = 'Elective'
ORDER BY id, total_marks DESC
) as b
WHERE rank <= 3
GROUP BY id
) AS e_marks ON t.id = e_marks.id
GROUP BY t.id, Elective_Marks;