题目
表：Enrollments

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| student_id    | int     |
| course_id     | int     |
| grade         | int     |
+---------------+---------+

(student_id, course_id) 是该表的主键。

编写一个 SQL 查询，查询每位学生获得的最高成绩和它所对应的科目，若科目成绩并列，取 course_id 最小的一门。查询结果需按 student_id 增序进行排序。

查询结果格式如下所示：

Enrollments 表：

+------------+-------------------+
| 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    |
+------------+-----------+-------+

Result 表：

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

解答
先选出每个学生的最高成绩

select E.student_id, max(E.grade) as max_grade
from Enrollments as E
group by E.student_id;

用二元in选择出最高成绩对应的情况

select E1.student_id, E1.course_id, E1.grade
from Enrollments as E1
where (E1.student_id, E1.grade) in (select E.student_id, max(E.grade) as max_grade
from Enrollments as E
group by E.student_id);

同分时 对id grade 分组 取最小的课程id即可

select E1.student_id, min(E1.course_id) as course_id, E1.grade
from Enrollments as E1
where (E1.student_id, E1.grade) in (select E.student_id, max(E.grade) as max_grade
from Enrollments as E
group by E.student_id)
group by E1.student_id, E1.grade;

其实对id分组即可

select E1.student_id, min(E1.course_id) as course_id, E1.grade
from Enrollments as E1
where (E1.student_id, E1.grade) in (select E.student_id, max(E.grade) as max_grade
from Enrollments as E
group by E.student_id)
group by E1.student_id
order by E1.student_id;