MySQL数据库设计的在线视频教育网站

I am planning on a website that offers users training videos on multiple subjects, users can view videos, check their view history, comment on videos, taking relative exams and gain badges/certificates if they succeed in the exam. Also they can submit live questions when they had a problem during the video. ALso there will be a forum where users can help each other out and add friends, send messages, etc. WIth this idea in mind, I am now designing the database, but I had no experience in database design before, so please review the following design and give me some suggestions on performance and best practices. Thank you all in advance. I am planning on a website that offers users tr