SQL & PL/SQL :: Trigger Sum Constraints

Basically, I need to fulfil the constraint:

"student cannot have more than 100 points"

I need to use a trigger to do this, but I don't understand how this would work.

Written in plain english I imagine it work work something like:

AFTER UPDATE
WHERE student.lessonid = lesson.lessonid
IF SUM(lesson.points) > 100 then ROLLBACK

lessonid is the only field relating the two tables.

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How to fulfill that criteria, whether it be using triggers or not.