Three tables courses,registration,students

columns in students

firstname,lastname,studentid,major,admitdate,graddate,gender,dob

columns in registration

courseid,studentid

columns in courses

coursenumber,coursename,credits

select statement I need to modify

select lastname as 'Last Name',sum(credits) as 'Credits Registered For' from students   as s
inner join registration as r on s.studentid = r.studentid
inner join courses as c on c.coursenumber = c.courseid
group by last name;

the question on the lab is... Modify the previous query to show all students, even if they have not registered for a class. You should have 14 rows. Students who are not registered will show NULL in output.

I know this requires outer join of some sort but I'm not fully grasping these joins i've read multiple posts on here and other sites but can't seem figure it out.