Prueba con
SELECT mdl_user.id ,mdl_course.fullname ,mdl_course_categories.name ,mdl_course_categories.description ,mdl_user.idnumber ,mdl_user.firstname ,mdl_user.lastname FROM mdl_user INNER JOIN mdl_role_assignments ON (mdl_user.id = mdl_role_assignments.userid) INNER JOIN mdl_context ON (mdl_role_assignments.contextid = mdl_context.id) INNER JOIN mdl_course ON (mdl_context.instanceid = mdl_course.id) INNER JOIN mdl_course_categories ON (mdl_course.category = mdl_course_categories.id) WHERE mdl_context.contextlevel = 50 and mdl_role_assignments.roleid = 5
post original