SQL : retrieving data by rank of variables - sql

Question

I have a dataset like this
student_id, course_id, grade
1 , 1, 2
1, 2, 5
1, 3 ,5
2, 3, 5
2, 1, 2
3, 1, 1
3, 2, 4
I created a schema for this on sqlfiddle.com like below:
CREATE TABLE enrollments(
STUDENT_ID INT NOT NULL,
COURSE_ID INT NOT NULL,
GRADE INT NOT NULL
);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(1, 1, 2);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(1, 2, 5);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(1, 3, 5);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(2, 3, 5);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(2, 1, 2);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(3, 1, 1);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(3, 2, 4);
INSERT INTO enrollments
(STUDENT_ID,COURSE_ID,GRADE) VALUES
(3, 3, 4);
Now here is what I want:
A query that returns the table with columns student_id, course_id, grade and which contains only the rows of the table corresponding to the highest grade each student was able to achieve across any of his/her courses.
If a student achieves the same highest grade in multiple courses, then only display the row corresponding to the course with the lowest course_id. Sort the output by student_id.
So I wrote the following query:
select STUDENT_ID, COURSE_ID, GRADE
from
(
select STUDENT_ID, rank() over(PARTITION BY STUDENT_ID ORDER BY GRADE Desc)
as grade_rank,
rank() over(PARTITION BY STUDENT_ID ORDER BY COURSE_ID asc) as course_rank
from enrollments
) as ss
where grade_rank=1 and course_rank=1
I want to test if this is the right logic on sqlfiddle but it throws an error for the query
ERROR: column "course_id" does not exist Position: 20
The schema has been successfully created there.
Is something wrong with this and how I can test if this is correct logic. If the logic is wrong, please highlight the error in code.
Thanks

Answer 1

You have to select the columns in the inner query too, if you want to select them in the outer query. Additionally have to use one RANK() with an ORDER BY regarding both columns.
SELECT STUDENT_ID,
COURSE_ID,
GRADE
FROM (SELECT STUDENT_ID,
COURSE_ID,
GRADE,
rank() OVER (PARTITION BY STUDENT_ID
ORDER BY GRADE DESC,
COURSE_ID ASC) R
FROM ENROLLMENTS) SS
WHERE R = 1;