MySQL：在 window function 中使用 MAX 不返回最高值

Question

I am seeking to find the highest grade scores for each student_id. The grade scores are kept in this table:

Enrollments table:

student_id	course_id	grade
2	2	95
2	3	95
1	1	90
1	2	99
3	1	80
3	2	75
3	3	82

The correct result should be:

student_id	course_id	grade
1	2	99
2	2	95
3	3	82

Here is the query I am trying:

SELECT student_id, course_id, grade
 FROM Enrollments
 GROUP BY student_id
 HAVING grade = (SELECT MAX(grade) OVER(PARTITION BY student_id))
 ORDER BY student_id ASC    

However with this query, the results I'm getting are:

student_id	course_id	grade
1	2	90
2	2	95
3	3	80

ie student 1's grade is 90, when it should be 99, and student 2's grade is 80, when it should be 82.

It seems like the issue is this query is only selecting the max grade from the first course_id in the table, but I'm not sure why that would be the case.

Answer 1

Please try this query.

SELECT e.student_id, e.course_id, e.grade
FROM Enrollments e INNER JOIN 
(SELECT student_id as student_id, MAX(grade) as grade FROM Enrollments GROUP BY student_id) sbq 
ON e.student_id = sbq.student_id
WHERE e.grade = sbq.grade;

Answer 2

You can use row_number, for your desired result

CREATE TABLE Table1 (`student_id` int, `course_id` int, `grade` int);

 INSERT INTO Table1 (`student_id`, `course_id`, `grade`) VALUES (2, 2, 95), (2, 3, 95), (1, 1, 90), (1, 2, 99), (3, 1, 80), (3, 2, 75), (3, 3, 82);

 SELECT `student_id`, `course_id`, `grade` FROM (SELECT `student_id`, `course_id`, `grade`, ROW_NUMBER() OVER(PARTITION BY `student_id` ORDER BY `grade` DESC) rn FROM Table1) t1 WHERE rn= 1

 student_id | course_id |  grade ---------: | --------: |  ----: 1 |  2 |  99 2 |  2 |  95 3 |  3 |  82 

db<>fiddle here