查找参加课程最多的学生 SQL

Question

假设有一个学生表，其中包含学生 ID、学期 ID 和每学期修读的课程数量：

student    semester    num_courses
10001      23          3
10002      23          5
10003      23          1
10004      25          2
10005      25          3
10003      25          5
10000      26          4
10013      26          2
...

我怎样才能找到每学期参加最多课程的学生？

预期的 output 将是：

student    semester    num_courses
10002      23          5
10003      25          5
10000      26          4
...

我想过使用OVER PARTITION BY ，但我不确定如何正确使用这种类型的查询。 我得到的没有按预期工作。

SELECT s.student, s.semester, MAX(s.num_courses) OVER (PARTITION BY s.semester) AS max
FROM students s; 

Answer 1

你的想法很好。 现在您有了学生的学期课程数和最大学期课程数，比较两者：

SELECT semester, num_courses, student
FROM
(
  SELECT
    student,
    semester, 
    num_courses,
    MAX(num_courses) OVER (PARTITION BY semester) AS semester_max_num_courses
  FROM students s
) with_max
WHERE num_courses = semester_max_num_courses
ORDER BY semester, student;

另一种方法是 select 所有最大学期课程计数，然后使用它来获取学生：

SELECT semester, num_courses, student
FROM students
WHERE (semester, num_courses) IN
(
  SELECT semester, MAX(num_courses)
  from students
  GROUP BY semester
)
ORDER BY semester, student;

Answer 2

使用 row_number() 您可以轻松地获得以 num_courses 降序排列的序列号。 然后只需为每个半学期选择具有第一个序列号的行。

子查询：

select student ,semester, num_courses from 
(
Select student ,semester, num_courses, row_number()over(partition by semester 
order by num_courses desc)rn from students 
)t
where rn=1

公用表表达式

With cte as
(
Select student ,semester, num_courses, row_number()over(partition by semester 
order by num_courses desc)rn from students 
)
select student ,semester, num_courses from cte where rn=1

Answer 3

另一种方法，如果一个以上的学生有最大数量的课程

WITH cte as 
( 
 SELECT s.semester, MAX(s.num_courses) as num
 FROM students s
 GROUP BY s.semester 
)
SELECT s.student, s.semester, s.num_courses FROM students s JOIN cte on s.semester = 
cte.semester
WHERE cte.num = s.course
ORDER BY s.semester

Answer 4

如果你想要每学期一排，那么在 Postgres 中，我会推荐distinct on ：

select distinct on (semester) s.*
from students s
order by semester, num_courses desc;

如果您希望所有学生人数最多，那么 Thorsten 的答案是合适的。