Question

我正在尝试使用PHP和Mysql创建论坛。我有这两个表：

论坛主题

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| project     | int(11)      | YES  | MUL | NULL    |                |
| title       | varchar(255) | NO   |     | NULL    |                |
| description | varchar(255) | NO   |     | NULL    |                |
| created     | datetime     | YES  |     | NULL    |                |
| is_locked   | tinyint(1)   | NO   |     | 0       |                |
| ForumTopic  | int(11)      | YES  | MUL | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

和论坛评论

+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| usr        | int(11)      | YES  | MUL | NULL    |                |
| content    | varchar(255) | NO   |     | NULL    |                |
| created    | datetime     | YES  |     | NULL    |                |
| is_deleted | tinyint(1)   | NO   |     | NULL    |                |
| ForumTopic | int(11)      | YES  | MUL | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

有根主题，包含子主题。子主题只能包含注释。如果ForumTopic（在Forum_topic表中，是的，我知道错误的命名）列是null，那么它是一个Root。否则，它包含根论坛主题的ID。

我需要的是一个查询，它输出每个根类别中的主题数量（类别=根主题），每个根类别的评论数量（表示所有儿童主题的评论）和最后一行未删除类别的注释（is_deleted = 0）。到目前为止我所拥有的是：

SELECT ID as topicId
     , (select count(*) 
          FROM `forum_topic` 
         where ForumTopic = topicId) as TopicCount
     , (SELECT count(*) 
          FROM `forum_comment` 
         where ForumTopic in (SELECT ID 
                                FROM `forum_topic` 
                               where ForumTopic = topicId) 
         ORDER 
            BY created desc) as postNumber
     , (SELECT ID FROM `forum_comment` 
         where ForumTopic in (SELECT ID 
                                FROM `forum_topic` 
                               where ForumTopic = topicId) 
         ORDER 
            BY created desc limit 1) as lastPostId 
  FROM `forum_topic` 
 where ForumTopic IS NULL 
   and project is null

使用以下输出：

+---------+------------+------------+------------+
| topicId | TopicCount | postNumber | lastPostId |
+---------+------------+------------+------------+
|       1 |          5 |         15 |         43 |
|       2 |          1 |          2 |         13 |
|       3 |          8 |          2 |         30 |
|       4 |          0 |          0 |       NULL |
+---------+------------+------------+------------+

我使用子查询来获得这个结果，这非常接近我需要的结果。但是，我真的需要具有该ID的行，而不是lastPostId（s）。所以我尝试加入了注释表并使用lastPostId作为一个列来加入，但它不起作用。这是我加入的尝试

SELECT forum_topic.ID as topicId, (select count(*) FROM `forum_topic` where ForumTopic = topicId) as TopicCount,(SELECT count(*) FROM `forum_comment` where ForumTopic in (SELECT ID FROM `forum_topic` where ForumTopic = topicId) ORDER BY created desc) as postNumber,(SELECT ID FROM `forum_comment` where ForumTopic in (SELECT ID FROM `forum_topic` where ForumTopic = topicId) ORDER BY created desc limit 1) as lastPostId 
FROM `forum_topic`
join forum_comment on lastPostId = forum_comment.id
where forum_topic.ForumTopic IS NULL and forum_topic.project is null

这会返回1054 - Unknown column 'lastPostId' in 'on clause'

我如何获得最后一个评论行？

编辑：示例数据可在此处找到： http://sqlfiddle.com/#!9/b65cfa/1

，预期结果应为。

Answer 1

您收到此错误是因为您加入的查询不是没有别名的表

所以你需要使用像这样的别名

包装你的查询

    (SELECT forum_topic.ID as topicId
        , (select count(*) FROM `forum_topic` where ForumTopic = topicId) as TopicCount
        ,(SELECT count(*) FROM `forum_comment` where ForumTopic in (SELECT ID FROM `forum_topic` where ForumTopic = topicId) ORDER BY created desc) as postNumber
        ,(SELECT ID FROM `forum_comment` where ForumTopic in (SELECT ID FROM `forum_topic` where ForumTopic = topicId) ORDER BY created desc limit 1) as lastPostId 
        FROM `forum_topic` where forum_topic.ForumTopic IS NULL and forum_topic.project is null) A

现在，您的查询中有了新的表A，并且您在A和forum_comment之间执行了连接

    Select A.topicId
        ,A.TopicCount
        ,A.postNumber
        ,A.lastPostId
            from (SELECT forum_topic.ID as topicId
                    , (select count(*) FROM `forum_topic` where ForumTopic = topicId) as TopicCount
                    ,(SELECT count(*) FROM `forum_comment` where ForumTopic in (SELECT ID FROM `forum_topic` where ForumTopic = topicId) ORDER BY created desc) as postNumber
                    ,(SELECT ID FROM `forum_comment` where ForumTopic in (SELECT ID FROM `forum_topic` where ForumTopic = topicId) ORDER BY created desc limit 1) as lastPostId 
                    FROM `forum_topic` where forum_topic.ForumTopic IS NULL and forum_topic.project is null) A

                join forum_comment on A.lastPostId = forum_comment.id

使用子查询结果

1 个答案: