MySQL/PDO-从其他表中选择(Join语句)

我有一个PDO语句,它从一些不同的表中选择一些数据。虽然,我不确定如何从另一个表中选择数据:

SELECT 
        c.forum_id as category_id,
        c.forum_name as category_name,
        t.forum_id as id,
        t.forum_name as name,
        t.forum_desc as description,
        (SELECT COUNT(*) FROM forum_topics WHERE forum_id=t.forum_id AND topic_deleted=0) as topics_count,
        (SELECT COUNT(*) FROM forum_posts WHERE forum_id=t.forum_id AND post_deleted=0) as posts_count,
        (SELECT COUNT(*) FROM forum_posts WHERE topic_id=lp.topic_id AND post_deleted=0) as last_post_count,
        lp.topic_id as last_post_topic_id,
        lp.topic_title as last_post_topic_title,
        lp.post_time as last_post_time,
        lp.username as last_post_username
    FROM forum_cats as t
    JOIN forum_cats as c on c.forum_id = t.forum_type_id
    left join (
        SELECT 
            ft.topic_id,
            ft.title as topic_title,
            tmp.post_time,
            u.username,
            fp.forum_id
        FROM
            forum_posts fp
            join forum_topics ft on ft.topic_id = fp.topic_id
            join users u on u.id = fp.userid
            join (
                select forum_id, max(`post_time`) `post_time`
                from forum_posts fp
                where fp.post_deleted = 0
                group by forum_id
                ) as tmp on (fp.forum_id = tmp.forum_id and fp.post_time = tmp.post_time)
        where post_deleted = 0 and ft.topic_deleted = 0
    ) as lp on lp.forum_id = t.forum_id
    where t.forum_active = 1 and c.forum_active = 1
    order by category_id, t.forum_id
");

现在,我要选择列 avatar . users 哪里 username . 用户 是= last_post_username ( 阿凡达 列,从 用户 表)。

我完全不知道从哪里开始。我应该这样写吗?

(SELECT avatar FROM users WHERE username=last_post_username)

但这不起作用。

感谢任何帮助。