if isnull in mysql select statement

I have the following sql. I want to be able to pull in a yes/no (true/false) if a subquery returns anything.

SELECT
              post.topic_id,
              topic.topic_posts,
              topic.topic_title,
              topic.topic_poster_name,
              topic.topic_last_post_id,
              topic.topic_start_time,
              (SELECT post_time FROM bb_posts WHERE post_id = topic.topic_last_post_id) as last_post_time,
              topic.topic_slug,
              topic.topic_posts,
              `group`.name AS group_name,
              `group`.slug AS child_slug,
              `parent`.slug AS parent_slug,
                      (SELECT id FROM table WHERE condition = 1) as isTrue << subquery
            FROM bb_posts post
            LEFT JOIN bb_topics topic
              ON topic.topic_id = post.topic_id
            LEFT JOIN bb_forums forum
              ON topic.forum_id = forum.forum_id
            LEFT JOIN wp_bp_groups_groupmeta groupmeta
    开发者_如何转开发          ON forum.forum_id = groupmeta.meta_value
            LEFT JOIN wp_bp_groups `group`
              ON groupmeta.group_id = `group`.id
            LEFT JOIN wp_bp_groups `parent`
              ON `group`.parent_id = `parent`.id
            $conditions
            GROUP BY topic_id
            ORDER BY $sort_col $dir
            LIMIT $offset,$num

I want the subquery to return a yes if results returned.

---

You can use count(*):

SELECT 0 != (SELECT COUNT(*) FROM (subquery))

Which gives you 1 if the subquery returned anything.

---

For the subquery to return a "yes" if results returned (and empty string if not):

SELECT IF(condition = 1, 'yes', '') AS isTrue FROM table WHERE some_condition_goes_here

That would work if you're joining on some column, which you don't specify in the subquery you showed. The join would go in the WHERE clause replacing "some_condition_goes_here". So something like:

WHERE table.id = some_other_table.tableid