Effectively modelling extended classes
You have supertype (content) which may be one of different suptypes (article, video,review). One approach to model such relationship in database is :

CONTENT(content_id, content_type_id, PK(content_id), UNIQUE(content_type_id,content_id));

 ARTICLE(content_id, content_type_id, 
  other attributes, 
  PK(content_type_id,content_id), 
  FK(content_type_id,content_id), CHECK (content_type_id = 'article_type_id'));

VIDEO(content_id, content_type_id, 
  other attributes, 
  PK(content_type_id,content_id), 
  FK(content_type_id,content_id), CHECK (content_type_id = 'video_type_id'));


etc. 

UNIQUE(content_type_id,content_id) seems redundant, but it's used to enforce storing of the sub-types in proper table, and only once. CHECK constraint restricts proper content type in each of detail tables. As far as I remember, Mysql still doesn't have unique CHECK constraints, but you can use enum to emulate them.