What is Self Join and why is it required?

Question

Forums

ID
Name
Description

Questions

ID
UserID
ForumID
Title
QuestionText
CreateDateTime

Replies

ID
UserID
QuestionID
ReplyText
CreateDateTime

users

UserID
UserName

I want to retrieve name of each forum, number of questions of each forum, number of replies of each forum, last post's UserName and date for each forum.

Answer 1

Self Join is the act of joining one table with itself.
Self Join is often very useful to convert a hierarchical structure into a flat structure

Example:
SELECT e.name EMPLOYEE, m.name MANAGER
FROM EMPLOYEE e, EMPLOYEE m
WHERE e.mgr_id = m.id (+)