Select columns other than the one specified in GROUP BY clause - mysql

Question

Is there a way to select columns other the one specified in the group by clause?
Let's say I have the following schema:
Student(id, name, age), Course(id, name, credit), Enrollment(student_id, course_id, grade)
I want to query for each course the following: course's name, student_count.
I came up with workaround, but I was wondering if there's a cleaner way to do this:
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

You might want to copy this DDL, adjust it to match your schema, and paste it into your question.
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');
Now, you can get the number of students enrolled in each course by querying only the "enrollment" table.
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
All that remains is to get the corresponding course name. To do that, you just join the table "Course" with the query we just wrote.
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

Answer 2

No, you can't. But you can extend GROUP BY with c.name:
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, c.name
Because e.course_id is unique, it won't change results.