在一个查询中查询4个MySQL表

Question

I'm building a forums site and I would like to display all the threads to users in the following format:

_________________________________________________________________________
Thread Title
Last Post: Username at Date, Started By: Username at Date
_________________________________________________________________________

This is what I've come up with:

         SELECT TD.id       thread_id,
                TD.sub_cat  thread_cat,
                TD.title    thread_title,
                TD.date     thread_date,
                TD.status   thread_status,
                TD.stick    thread_stick,

                US.username user,

                GR.color group_color

        FROM fr_thread AS TD
        INNER JOIN user AS US ON TD.user_id = US.id
        INNER JOIN user_group AS GR ON US.user_group = GR.id
        ORDER BY TD.stick

While I'm displaying all the threads, I would also like the username's be to coloured according to what user group they are in. Therefore, I also have to query for that.

Although, I've come up with most of query, I'm stuck as how to query for Username, group color, date of the last poster.

Here is roughly what my DB design looks like:

user:
id
user_group

user_group:
id
color

fr_thread:
id
sub_cat
user_id
date

fr_reply:
id
thread_id
user_id
date

Answer 1

Try

SELECT  

    TD.id as thread_id , 
    TD.sub_cat as thread_sub_cat , 
    TD.user_id as thread_started_user_id , 
    TD.date as thread_started_on , 

    US1.name as thread_author , 
    US2.id as last_reply_user_id , 
    US2.name as last_reply_user_name , 

    USG1.color as thread_author_color ,  
    USG2.color as lat_reply_user_color , 

    RE.date as last_reply_date 

FROM  

    fr_thread TD join user US1 on TD.user_id = US1.id

    join user_group USG1 on USG1.id = US1.user_group 

    join fr_reply RE on RE.thread_id = TD.id

    join user US2 on US2.id = RE.user_id 

    join user_group USG2 on USG2.id = US2.user_group

Created 2 aliases for user table as US1 and US2 and there are 2 involved ( creator and last replier) similary for user_group

http://sqlfiddle.com/#!9/951c0b/1