I’ll try to explain how a join on more than one table works. I’ve noticed people get confused by it. Assume we have the following tables:
- newsitems(news_id,post_id)
- postitems(post_id,user_id,content)
- users(user_id,name,password)
We want to display for each newsitem the content and the author.
Our base table would be the newsitems, and then we join using the item_id. Thus the query would be:
SELECT * FROM newsitems INNER JOIN ON postitems USING (post_id)
This returns a “virtual table” that has looks like this result(news_id,post_id,user_id,content).
Now we still need to get the username, so we use our result table and perform a join on the users table. Thus the query would be:
SELECT * FROM result INNER JOIN users USING (user_id)
If we combine our first two queries, we end up with this:
SELECT * FROM newsitems INNER JOIN postitems USING (post_id) INNER JOIN users USING (user_id)
Conclusion: Look at A*B*C as (A*B)*C to easily understand multiple joins