SQL MAX() grouping - sql

Question

Tables:
student(sid, sname, sex, age, year, gpa)
major(dname, sid)
Question:
For each department with more than 15 students majoring in the department, we want to print information about the student(s) with the highest GPA within the department. In particular, for each such student, we want to print the student id, student name and GPA, and the department name the student is major in.
So far I have:
SELECT student.sid, student.sname, student.gpa, major.dname
FROM student
RIGHT JOIN major ON student.sid = major.sid
WHERE student.gpa IN (
SELECT MAX(gpa)
FROM student JOIN major ON student.sid = major.sid
GROUP BY dname
HAVING COUNT(dname) > 15
)
But it doesn't give me the accurate query. The clause inside the IN works but when put together in this way it doesn't actually match student.gpa to dname max GPA. What am I doing wrong here?
This Query gives:
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I need:
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Answer 1

Your inner query gives you the maximum GPA for each department. Then the outer query returns all students who have a GPA equal to any of the maximum GPA's, regardless of the department. The quickest fix of your code is to use a correlated subquery, that will find the maximum GPA for the student's specific department.
SELECT s.sid, s.sname, s.gpa, sm.dname
FROM student s
RIGHT JOIN major sm ON s.sid = sm.sid
WHERE student.gpa IN (
SELECT MAX(ds.gpa)
FROM student ds JOIN major dm ON ds.sid=dm.sid
WHERE dm.dname = sm.dname
GROUP BY dm.dname
HAVING COUNT(dm.dname)>15
)

Answer 2

This query:
select dname, max(s.gpa) maxgpa
from major m inner join student s
on s.sid = m.sid
group by dname
having count(s.sid) > 15
returns all the departments with more than 15 students and the highest gpa in that department.
Join it to the 2 tables like this:
select s.sid, s.sname, s.gpa, t.dname
from (
select dname, max(s.gpa) maxgpa
from major m inner join student s
on s.sid = m.sid
group by dname
having count(s.sid) > 15
) t
inner join major m on m.dname = t.dname
inner join student s on s.sid = m.sid and s.gpa = t.maxgpa
Or with window functions:
select t.sid, t.sname, t.gpa, t.dname
from (
select m.dname, s.*,
rank() over (partition by m.dname order by s.gpa desc) rn,
count(s.sid) over (partition by m.dname) counter
from major m inner join student s
on s.sid = m.sid
) t
where t.counter > 15 and t.rn = 1

Answer 3

If you change your WHERE ... IN statement to be an INNER JOIN, you can connect more fields.
SELECT student.sid, student.sname, student.gpa, major.dname
FROM student
RIGHT JOIN major
ON student.sid = major.sid
INNER JOIN (
SELECT MAX(gpa) as max_gpa, dname
FROM student JOIN major ON student.sid=major.sid
GROUP BY dname
HAVING COUNT(dname)>15
) as dept_gpa
ON student.gpa = dept_gpa.max_gpa
AND major.dname = dept_gpa.dname