Joining tables based on the maximum value

Question

Here's a simplified example of what I'm talking about:

Table: students      exam_results
_____________       ____________________________________
| id | name |       | id | student_id | score |   date |
|----+------|       |----+------------+-------+--------|
|  1 | Jim  |       |  1 |          1 |    73 | 8/1/09 | 
|  2 | Joe  |       |  2 |          1 |    67 | 9/2/09 |
|  3 | Jay  |       |  3 |          1 |    93 | 1/3/09 |
|____|______|       |  4 |          2 |    27 | 4/9/09 |
                    |  5 |          2 |    17 | 8/9/09 |
                    |  6 |          3 |   100 | 1/6/09 |
                    |____|____________|_______|________|

Assume, for the sake of this question, that every student has at least one exam result recorded.

How would you select each student along with their highest score? Edit: ...AND the other fields in that record?

Expected output:

_________________________
| name | score |   date |
|------+-------|--------|
|  Jim |    93 | 1/3/09 |
|  Joe |    27 | 4/9/09 |
|  Jay |   100 | 1/6/09 |
|______|_______|________|

Answers using all types of DBMS are welcome.

Romanticist

How would you resolve ties? In your example, which record should be selected in Jim scored 93 twice? – Ela

in my own tables, (which are nothing to do with students and exams) this doesn't happen. Either one should be fine? – Romanticist

Answer 1

Answering the EDITED question (i.e. to get associated columns as well).

In Sql Server 2005+, the best approach would be to use a ranking/window function in conjunction with a CTE, like this:

with exam_data as
(
    select  r.student_id, r.score, r.date,
            row_number() over(partition by r.student_id order by r.score desc) as rn
    from    exam_results r
)
select  s.name, d.score, d.date, d.student_id
from    students s
join    exam_data d
on      s.id = d.student_id
where   d.rn = 1;

For an ANSI-SQL compliant solution, a subquery and self-join will work, like this:

select  s.name, r.student_id, r.score, r.date
from    (
            select  r.student_id, max(r.score) as max_score
            from    exam_results r
            group by r.student_id
        ) d
join    exam_results r
on      r.student_id = d.student_id
and     r.score = d.max_score
join    students s
on      s.id = r.student_id;

This last one assumes there aren't duplicate student_id/max_score combinations, if there are and/or you want to plan to de-duplicate them, you'll need to use another subquery to join to with something deterministic to decide which record to pull. For example, assuming you can't have multiple records for a given student with the same date, if you wanted to break a tie based on the most recent max_score, you'd do something like the following:

select  s.name, r3.student_id, r3.score, r3.date, r3.other_column_a, ...
from    (
            select  r2.student_id, r2.score as max_score, max(r2.date) as max_score_max_date
            from    (
                        select  r1.student_id, max(r1.score) as max_score
                        from    exam_results r1
                        group by r1.student_id
                    ) d
            join    exam_results r2
            on      r2.student_id = d.student_id
            and     r2.score = d.max_score
            group by r2.student_id, r2.score
        ) r
join    exam_results r3
on      r3.student_id = r.student_id
and     r3.score = r.max_score
and     r3.date = r.max_score_max_date
join    students s
on      s.id = r3.student_id;

EDIT: Added proper de-duplicating query thanks to Mark's good catch in comments

Russon

I don't think the distinct works to de-duplicate ties if the dates are different. – Untune

Good point Mark - would need to use something deterministic in another subquery to de-dupe correctly in the ANSI query. I'll edit to reflect... – Russon

Invalid column name 'score': I think you got a couple of table names mixed up. – Untune

Yeah, that's what you get when you write a query in a text editor...I just adjusted it, had the filters backwards on the final join conditions – Russon

Answer 2

SELECT s.name,
    COALESCE(MAX(er.score), 0) AS high_score
FROM STUDENTS s
    LEFT JOIN EXAM_RESULTS er ON er.student_id = s.id
GROUP BY s.name

Eringo

This assumes that there could be students without exams associated. – Eringo

ahhh ok, it seems i've asked the question poorly. I'll reword. – Romanticist

have to add a group by clause for this to work, i.e. "group by s.name" – Russon

Answer 3

Try this,

Select student.name, max(result.score) As Score from Student
        INNER JOIN
    result
        ON student.ID = result.student_id
GROUP BY
    student.name

Casavant

thanks Zinx, however I stuffed up the original wording of the question. I actually need to know more than just the high score: I need to know all other fields in the record which holds their high score, too. – Romanticist

Zinx, highlight your code and hit Ctrl+K to format the query syntax, easier reading – Russon

Answer 4

With Oracle's analytic functions this is easy:

SELECT DISTINCT
       students.name
      ,FIRST_VALUE(exam_results.score)
       OVER (PARTITION BY students.id
             ORDER BY exam_results.score DESC) AS score
      ,FIRST_VALUE(exam_results.date)
       OVER (PARTITION BY students.id
             ORDER BY exam_results.score DESC) AS date
FROM   students, exam_results
WHERE  students.id = exam_results.student_id;

Olive

I know PostgreSQL isn't in the question list, but in case anyone stumbles across this, it has window functions to do this, too. – Passus

Answer 5

Select Name, T.Score, er. date 
from Students S inner join
          (Select Student_ID,Max(Score) as Score from Exam_Results
           Group by Student_ID) T 
On S.id=T.Student_ID inner join Exam_Result er
On er.Student_ID = T.Student_ID And er.Score=T.Score

Counterclockwise

Answer 6

Using MS SQL Server:

SELECT name, score, date FROM exam_results
JOIN students ON student_id = students.id
JOIN (SELECT DISTINCT student_id FROM exam_results) T1
ON exam_results.student_id = T1.student_id
WHERE exam_results.id = (
    SELECT TOP(1) id FROM exam_results T2
    WHERE exam_results.student_id = T2.student_id
    ORDER BY score DESC, date ASC)

If there is a tied score, the oldest date is returned (change date ASC to date DESC to return the most recent instead).

Output:

Jim 93  2009-01-03 00:00:00.000
Joe 27  2009-04-09 00:00:00.000
Jay 100 2009-01-06 00:00:00.000

Test bed:

CREATE TABLE students(id int , name nvarchar(20) );

CREATE TABLE exam_results(id int , student_id int , score int, date datetime);

INSERT INTO students
VALUES
(1,'Jim'),(2,'Joe'),(3,'Jay')

INSERT INTO exam_results VALUES
(1, 1, 73, '8/1/09'), 
(2, 1, 93, '9/2/09'),
(3, 1, 93, '1/3/09'),
(4, 2, 27, '4/9/09'),
(5, 2, 17, '8/9/09'),
(6, 3, 100, '1/6/09')

SELECT name, score, date FROM exam_results
JOIN students ON student_id = students.id
JOIN (SELECT DISTINCT student_id FROM exam_results) T1
ON exam_results.student_id = T1.student_id
WHERE exam_results.id = (
    SELECT TOP(1) id FROM exam_results T2
    WHERE exam_results.student_id = T2.student_id
    ORDER BY score DESC, date ASC)

On MySQL, I think you can change the TOP(1) to a LIMIT 1 at the end of the statement. I have not tested this though.

Untune

Answer 7

select  s.id, s.name, e.score, e.date
from    students s
join    exam_result e      on      s.id = e.student_id
join    exam_result e1     on      s.id = e1.student_id
group by s.id, s.name, e.date
having e.score = max(e1.score);

you can try this. It suits me

Bowlds