Tips for building complex SQL queries - mysql

Question

I have started programming for sometime with PHP using MySQL as the database and I know basic SQL queries upto simple JOINS containing upto two tables.
But the moment I need to get results from 3 or more tables, I am stuck. No matter how hard I try, I still manage to find myself lost. I have searched everywhere looking for a good tutorial on how to tackle complex SQL queries but haven't found anything that explains how to go about it. Most tutorials consists of solutions for a particular problem, but they don't explain the perfect general procedure how to go about tackling a problem
Can anyone explain the basic general way of going about from start to finish, how to construct the query, etc when it comes to complex queries.
For example:
I have a forum with the following database structure:
forumCategory:
id | name | desc
forumTopic:
id | category_id | created_on | created_by | title | content
forumPost:
id | topic_id | created_on | created_by
users:
id | first_name | last_name
All topics are created in the forumTopic table. All replies to that topic are inserted in the forumPost table.
Now on the forum main page, I need to display the categories, the very last post posted by a user in that particular category, the user who posted the last post.
The flow I thought of was :
Find the last post in a category by looking at the MAX(id) in the forumPost table grouped by topic_id. This will get me the ID of the last post in every topic.
Now again find the MAX(of the IDS I got earlier) grouped by category_id. This will get me the last post in every category.
IDs are autoincrementing primary keys.
But I got stuck with constructing a SQL query from the above algorithm
Would be very helpful if someone could help me out on this one.

Answer 1

Get the last post for each category by joining Post to Topic
SELECT category_id , category.name, Max(ForumPost.ID) as maxpostid
from ForumPost
inner join ForumTopic on ForumPost.Topic_ID = ForumTopic.ID
inner join ForumCategory on ForumTopic.Category_Id = ForumCategory.ID
group by category_Id, category.name
(This is an intermediate stage for explanatory purposes - it is included in the query below )
Then join this to the user table to find out the user name (presumably a post has a userid?)
select users.name, lastposts.*
from
forumpost
inner join
(
SELECT category_id , category.name, Max(ForumPost.ID) as maxpostid
from ForumPost
inner join ForumTopic on ForumPost.Topic_ID = ForumTopic.ID
inner join ForumCategory on ForumTopic.Category_Id = ForumCategory.ID
group by category_Id, category.name
) lastposts
on forumpost.id = lastposts.maxpostid
inner join
users on forumpost.userid =users.id
However, you may want to consider updating the category table with the last post each time a post is made. That way you can run a much simpler query for your forum front page.