How can I write a query to do this? - sql

Question

I'm working with this database: link.
I want to write a query to get the student name who belongs to class 2 (class_id = 2) and has the highest average, and the highest degree in a specific course(course_id = 9) at the same time, and if there is no student with these data then return nothing.
I managed to write half of it and get the student with the highest average in class 2, but I didn't know how to add the 2nd condition(highest degree in course 9), I tried in many ways it didn't work.
This is my query: (sorry for the messy code, I'm still learning)
SELECT s.studentname
FROM students s
JOIN section se
ON s.sectionid = se.sectionid
JOIN courses_student cs
ON s.student_id = cs.student_id
WHERE se.classes_id = 2
GROUP BY s.studentname
HAVING ( Avg(cs.exam_season_one)
+ Avg(cs.exam_season_two)
+ Avg(cs.degree_season_one)
+ Avg(cs.degree_season_two) ) / 4 = (SELECT
Max(( Avg(cs.exam_season_one)
+ Avg(cs.exam_season_two)
+ Avg(cs.degree_season_one)
+ Avg(cs.degree_season_two)
) / 4)
FROM students s
JOIN section se
ON s.sectionid =
se.sectionid
JOIN courses_student cs
ON s.student_id =
cs.student_id
WHERE se.classes_id = 2
GROUP BY s.studentname)
The way I calculate the average is:
( Avg(cs.exam_season_one)
+ Avg(cs.exam_season_two)
+ Avg(cs.degree_season_one)
+ Avg(cs.degree_season_two) ) / 4
When the way I calculate the degree in a specific course is:
(cs.exam_season_one
+ cs.exam_season_two
+ cs.degree_season_one
+ cs.degree_season_two ) / 4
So how can I edit my query to add the second condition? or is there a better way than this?
PS: I use oracle

Answer 1

You can find the maximum and the average for each student and then use analytic functions to find the greatest for both conditions:
SELECT studentname
FROM (
SELECT studentname,
ROW_NUMBER() OVER (ORDER BY avg_degree DESC) AS rn_avg,
ROW_NUMBER() OVER (ORDER BY max_degree DESC) AS rn_max
FROM (
SELECT MAX(s.studentname) AS studentname,
AVG(
cs.exam_season_one + cs.exam_season_two + cs.degree_season_one + cs.degree_season_two
) / 4 AS avg_degree,
MAX(
CASE
WHEN cs.courses_id = 9
THEN cs.exam_season_one + cs.exam_season_two + cs.degree_season_one + cs.degree_season_two
END
) / 4 AS max_degree
FROM students s
JOIN section se
ON s.sectionid = se.sectionid
JOIN courses_student cs
ON s.student_id = cs.student_id
WHERE se.classes_id = 2
GROUP BY s.student_id
)
)
WHERE rn_avg = 1
AND rn_max = 1;
Again, don't GROUP BY studentname as you can have two students with the same name.
Which outputs:
STUDENTNAME
maher
sqlfiddle here