mysql - 选择 GROUP BY 子句中指定的列以外的列

Question

有没有一种方法可以选择 group by 子句中指定的列以外的列？ 假设我有以下架构:

学生(id、姓名、年龄)、类(class)(id、姓名、学分)、入学情况(student_id、course_id、年级)

我想查询每门类(class)的以下信息:类(class)名称、student_count。

我想出了解决方法，但我想知道是否有更干净的方法来做到这一点:

SELECT MAX(c.name), COUNT(distinct e.student_id)
FROM Enrollment e
INNER JOIN Course c ON c.id = e.course_id
GROUP BY e.course_id;

Answer 1

您可能需要复制此 DDL，调整它以匹配您的架构，然后将其粘贴到您的问题中。

create table Student(
  student_id integer primary key, 
  student_name varchar(35) not null, 
  age int not null default 20
);

create table Course(
  course_id integer primary key, 
  course_name varchar(35) not null, 
  credit integer not null default 3
);

create table Enrollment(
  student_id integer not null references Student (student_id), 
  course_id integer not null references Course (course_id), 
  primary key (student_id, course_id),
  grade char(1) not null
);

insert into student values 
(1, 'a', 20),
(2, 'b', 20),
(3, 'c', 20);

insert into course values
(1, 'course 1', 3),
(2, 'course 2', 3),
(3, 'course 3', 3);

insert into enrollment values
(1, 1, 'b'),
(2, 1, 'b'),
(3, 1, 'b'),
(1, 2, 'b'),
(2, 2, 'b'),
(3, 3, 'b');

现在，您只需查询“注册”表即可获取每门类(class)注册的学生人数。

select course_id, count(student_id) num_students
from enrollment
group by course_id
order by course_id;

course_id  num_students
--
1          3
2          2
3          1

剩下的就是获取相应的类(class)名称。为此，您只需将表“Course”与我们刚刚编写的查询连接起来即可。

select course.course_name, course_enrollment.num_students
from course
inner join (select course_id, count(student_id) num_students
            from enrollment
            group by course_id) course_enrollment
        on course.course_id = course_enrollment.course_id;

course_name  num_students
--
course 1     3
course 3     1
course 2     2