Deleting students that have no courses

[amrigo]

Hi

I have a students table and i need to clean the table from the students that have no courses

The enrolment happens at the role_assignments table the course key is related to the context table wich is related to the role-assignments table

And also just the students with the auth value = db

user
----
id
name

role
----
id
name

context
-------
id
instanceid (relates to course)

course
-------
id
fullname

role_assignments
----------------
roleid (relates to role: "student, teacher")
userid(relates to user)
contextid (relates to context)

This query brings the users who have any enrolment in a course

SELECT u.email from user u
inner join role_assignments ra on ra.userid=u.id
inner join role r on r.id=ra.roleid
inner join context ct on ra.contextid=ct.id
inner join course c on c.id=ct.instanceid
WHERE u.auth='db' AND r.id=5



Thank´s inadvance