SQL get duplicate records for all the users - sql

Question

I have these tables
CREATE TABLE subject
(
id int auto_increment primary key,
name varchar(20)
);
INSERT INTO subject
(name)
VALUES
('subject 1'),('subject 2'), ('subject 3');
CREATE TABLE course
(
id int auto_increment primary key,
name varchar(20),
subject_id int,
FOREIGN KEY (subject_id) REFERENCES subject(id)
);
INSERT INTO course
(subject_id, name)
VALUES
(1, 'course 1'),(1, 'course 2'), (2, 'course 3'), (3, 'course 4');
CREATE TABLE user
(
id int auto_increment primary key,
name varchar(20),
course_id int,
FOREIGN KEY (course_id) REFERENCES course(id)
);
INSERT INTO user
(course_id, name)
VALUES
(1, 'User 1'),(1, 'User 1'), (2, 'User 3'), (3, 'User 4');
I want to get a list of users who belong to the course which belongs to the same subject.
I ran this query to get the list
select u.name as user_name, s.name as subject_name from user u
inner join course c on u.course_id = c.id
inner join subject s on c.subject_id = s.id;
And this is the output
USER_NAME SUBJECT_NAME
User 1 subject 1
User 1 subject 1
User 3 subject 1
User 4 subject 2
Expected output
USER_NAME SUBJECT_NAME
User 1 subject 1
User 1 subject 1
how to I select only user 1 who has a course that belongs to the same subject ??
fiddle link

Answer 1

To get the List of users who have got more than 1 course enrolled, use below query in the fiddle:
select u.name as user_name, s.name as subject_name from user u
inner join course c on u.course_id = c.id
inner join subject s on c.subject_id = s.id
having count(1) > 1;
To Get only distinct users and courses, use below query :
select distinct u.name as user_name, s.name as subject_name from user u
inner join course c on u.course_id = c.id
inner join subject s on c.subject_id = s.id;
EDIT: I assume you need the number of distinct users who have taken the subject for which you can use below query:
select s.name as subject_name, count(distinct(u.name)) as no_of_users from user u
inner join course c on u.course_id = c.id
inner join subject s on c.subject_id = s.id
group by s.name;
By using just count(u.name) as no_of_users You would get the actual count rather than distinct count.
EDIT AGAIN
To get the required output, the below query works!
select tab_a.user_name, tab_a.subject_name from
(
select u.name as user_name, s.name as subject_name from user u
inner join course c on u.course_id = c.id
inner join subject s on c.subject_id = s.id
)tab_a
inner join
(
select u.name as user_name, s.name as subject_name, count(1) reccount from user u
inner join course c on u.course_id = c.id
inner join subject s on c.subject_id = s.id
group by u.name, s.name
) tab_b
on tab_a.user_name = tab_b.user_name
where tab_b.reccount >1