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sql - Joining tables based on the maximum value

问题描述:

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.

网友答案:

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

网友答案:
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
网友答案:

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
网友答案:

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;
网友答案:

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.

网友答案:
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
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