Postgresql getting other information from max tuple

Question

Say you have a table with studentID, class, grade. I want the maximum grade for each class. This is easy, just group by class and get max(grade). But the problem I'm having is how to also get the studentID.

Answer 1

Instead of using an aggregate function, you could use window functions:

SELECT class, grade, studentId
FROM   (SELECT class, grade, studentId, 
               RANK() OVER (PARTITION BY class ORDER BY grade DESC) rk
        FROM   students)
WHERE  rk = 1

Answer 2

I think distinct on is a good way to go:

select distinct on (s.class) s.*
from students
order by s.class, s.grade desc;

However you probably want all students for each class with the maximum grade. If so, Mureinik's solution is better.