Question

I am trying to write a query to find out what users are not enrolled on some courses.

I have 2 tables; courses, and users. Both contain the fields ‘id’ and then ‘coursename’ and username’ respectively.

Using these two table the existing system adds users to a third table (called enrolled_users). Users take up one row for each. For example if user id ‘1’ is on 4 courses the enrolled_users looks like this:

-----------------------
| user_id | course_id |
-----------------------
|     1   |     1     |
|     1   |     2     |
|     1   |     3     |
|     1   |     4     |
-----------------------

As you can see this table data is not built on a relationship, it is generated in PHP using the two tables ‘courses’ and ‘users’.

I am not sure how to write a query on how to find out that courses user ‘1’ is not enrolled in, if there are say 10 courses, but we can see user ‘1’ is only on courses 1-4.

Am I suppose to use something like a LIKE or NOT LIKE?

SELECT u.id, c.id, ec.user_id, ec.course_id
FROM users u, courses c, enrolled_courses ec
WHERE u.id = ec.user_id
AND c.id = ec.course_id
AND u.id = '1'

I have tried using != and <> but this doesn’t show what i need; a list of courses the user is not enrolled on.

Sorry if I am being vague, trying to get my head round it!

Using MySQL 5.0, on an existing system I cannot modify, only query from (for the moment).

Answer 1

SELECT
    *
FROM
    courses c
WHERE
    c.id NOT IN (
    SELECT
        ec.course_id
    FROM
        enrolled_courses ec
    WHERE
        ec.user_id = 1 AND ec.course_id IS NOT NULL
    )