创建触发器以更新表中的一列

I have two tables : courses and courses_partners. In table courses I have features of particular course and also field "public". In courses_partners I have course_id, partner_id , which are foreign keys (relation for M:N). I want to set "public" field in courses table to "n" when count of rows in courses_partners where course_id=1(for example) is maxAmount(what is read from database :

SELECT maxAmount FROM courses WHERE course_id=1

for example. I have to do it in mysql database with trigger.