Database design problem

Question

 i am developing an CMS for teachers , where they can login, design thier course and then post articles which will be save on SQL Server database.

1.Teacher Logs in 

2.He choose a Category (Let say Web Development)

3.Then he selects a Subcategory (say .. Javascript)

4.then he enters chapters in Javascript he want to teach.

5.then he enters Topics of each chapters.

6.Some Topics may have subtopics. 

7.Finally he writes post on a particular subtopic/Topic of a chapter for a course under subcategory of category.

 

my question is about the structure of Post table 

PostId

Title

Body

TopicId

ParentTopicId  /// it indicates i am not reating separate table for sub topic

 

 

 

OR

 

PostId

Title

Body

TopicId

SubTopicId   /// it indicates i am creating separate Table for subtopic

 

which one is good for faster lookup and maintanance or any better solution.