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 :

1. 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.

2. 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.