如果mysql select语句中为空

我有以下sql语句。如果子查询返回任何内容，我希望能够输入yes/no true/false

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
              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

如果返回结果，我希望子查询返回yes。

您可以使用count*：

选择0！=从子查询中选择COUNT*

---

如果子查询返回任何结果，则返回1。

如果返回结果，则子查询返回yes；如果返回结果，则返回空字符串：

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
              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

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

如果您要加入某个列，而您没有在显示的子查询中指定该列，那么这将起作用。连接将进入WHERE子句，替换此处的某些条件。比如：

WHERE table.id = some_other_table.tableid

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如何在子查询的WHERE条件中获取post.topic_id。在您的示例中，您似乎嵌套了一个子查询两次。我无法从父查询中获取post.topic\u id。