SQL COUNT Return Wrong Number - mysql

Question

so I have a problem with my query. I have 2 tables:
courses:
The user_id in this table is the instructor of the course.
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| course_id | user_id | course_name | other information |
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| 6 | 1 | My Course 1 | ... |
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my_courses:
The user_id in this table is a student of the course.
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| user_id | course_id | created_at |
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| 5 | 6 | [UNIX_TIMESTAMP] |
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The my_courses contains the number of people that have joined that course. I want to get all the course info as well as the number of people that have joined a course. Everything returns as expected except the number of people that joined a course. This is the query I'm using:
SELECT
courses.*,
users.name, //This is the name of the instructor
users.last_name, //This is the last name of the instructor
COUNT(my_courses.user_id) as count_students
FROM courses
LEFT JOIN users
ON courses.user_id = courses.user_id
LEFT JOIN my_courses
ON courses.course_id = my_courses.course_id
WHERE courses.course_id = '6'
Like I said, this query returns the course info like normal but returns 3 as count_students when it should only return 1. Does anyone know why this is happening? Any help is greatly appreciated.

Answer 1

It's probably because of the JOIN condition for users. It should be:
LEFT JOIN users
ON courses.user_id = users.user_id
You should also add a GROUP BY clause in your query:
SELECT
c.*,
u.name,
u.last_name,
COUNT(mc.user_id) AS count_students
FROM courses c
LEFT JOIN users u
ON c.user_id = u.user_id
LEFT JOIN my_courses mc
ON c.course_id = mc.course_id
WHERE c.course_id = '6'
GROUP BY
<columns not in the aggregate function>
Additionally, alias your tables to improve readability.

Answer 2

JOIN operations cause combinatorial multiplication of rows. You need to summarize the student count from its own table like so.
SELECT course_id, COUNT(*) students
FROM my_courses
GROUP BY course_id
That gives you a result set with either one or zero rows per course_id. You can then join it to the rest of your query.
SELECT courses.*,
users.name, //This is the name of the instructor
users.last_name, //This is the last name of the instructor
aggr.count_students
FROM courses
LEFT JOIN users ON courses.user_id = courses.user_id
LEFT JOIN (
SELECT course_id, COUNT(*) students
FROM my_courses
GROUP BY course_id
) aggr ON courses.course_id = aggr.course_id
WHERE courses.course_id = '6'
That way you'll avoid multiple-counting your students for courses with, perhaps, more than one instructor.