Question

我正在构建嵌套评论功能。我有submissions_comments看起来像：

+---------------+------------------+------+-----+---------+----------------+
| Field         | Type             | Null | Key | Default | Extra          |
+---------------+------------------+------+-----+---------+----------------+
| id            | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| user_id       | int(10) unsigned | NO   | MUL | NULL    |                |
| submission_id | int(11)          | NO   | MUL | NULL    |                |
| comment       | text             | NO   |     | NULL    |                |
| parent_id     | int(10) unsigned | YES  | MUL | NULL    |                |
| created       | datetime         | NO   | MUL | NULL    |                |
| created_ip    | int(11)          | NO   |     | NULL    |                |
| helpful_count | int(11)          | NO   | MUL | NULL    |                |
| deleted       | tinyint(4)       | NO   | MUL | 0       |                |
+---------------+------------------+------+-----+---------+----------------+'

和看起来像的用户：

  +----------------+------------------------+------+-----+-------------------+-----------------------------+
| Field          | Type                   | Null | Key | Default           | Extra                       |
+----------------+------------------------+------+-----+-------------------+-----------------------------+
| id             | int(10) unsigned       | NO   | PRI | NULL              | auto_increment              |
| email          | varchar(128)           | NO   | MUL | NULL              |                             |
| username       | varchar(23)            | NO   |     | NULL              |                             |
| name           | varchar(32)            | NO   |     | NULL              |                             |
| about          | varchar(255)           | NO   |     | NULL              |                             |
+----------------+------------------------+------+-----+-------------------+-----------------------------+

我正在使用GROUP_CONCAT和LEFT JOIN通过此查询向我提供所有儿童评论（在我的submissions_comments表格内）：

SELECT parent.id, MAX(parent.comment) as pcomm,
       GROUP_CONCAT(child.id ORDER BY child.id) as children,
       GROUP_CONCAT(child.comment ORDER BY child.id) as childrenComments
FROM   submissions_comments AS parent
 LEFT  JOIN submissions_comments AS child
   ON  child.parent_id = parent.id
WHERE  parent.parent_id IS NULL
GROUP  BY parent.id
ORDER  BY parent.id;

给了我：

+----+-------------------------------+----------+--------------------------------------------------------+
| id | pcomm                         | siblings | siblingComments                                        |
+----+-------------------------------+----------+--------------------------------------------------------+
|  1 | This is a parent              | 2,4      | This is a child comment,This is a second child comment |
|  3 | I don&#39;t have any children | NULL     | NULL                                                   |
|  5 | Testing one two three         | NULL     | NULL                                                   |
|  6 | adsdfsasdf                    | NULL     | NULL                                                   |
|  7 | asdfadsfdsaf                  | NULL     | NULL                                                   |
|  8 | asdfasdsadfsadf               | NULL     | NULL                                                   |
|  9 | asdfsdafsdafdaafds            | NULL     | NULL                                                   |
+----+-------------------------------+----------+--------------------------------------------------------+

我的问题：我正在users u加入submissions_comments，因此我可以获得username，email，{{1每个父评论和子评论都有，name。

我的查询：

about

我想要的是什么：

SELECT parent.id, MAX(parent.comment) as pcomm,
       parent.username, 
       GROUP_CONCAT(child.id ORDER BY child.id) as children,
       GROUP_CONCAT(child.comment ORDER BY child.id) as childrenComments,
       GROUP_CONCAT(child.username ORDER BY child.id) as childrenUsernames
FROM   submissions_comments AS parent
 LEFT  JOIN submissions_comments AS child
   ON  child.parent_id = parent.id
LEFT JOIN users u on parent.user_id = u.id
GROUP  BY parent.id
ORDER  BY parent.id;

我一直收到这些错误：ERROR 1054（42S22）：'字段列表'和ERROR 1054（42S22）中的未知列'parent.username'：'字段列表'中的未知列'child.username'。

Answer 1

parent是submissions_comments的别名，其中没有此列username。
只有users表有一个名为username的列。

变化：

select ...
parent.username,
...

要：

select ...
u.username,
...

您的group by子句中也有一个错误：您必须添加username才能获得正确的行为：

GROUP BY parent.id, u.username

Answer 2

如果您需要parent和child的用户名，那么您将不得不去获取它们;它们不在submissions_comments表中，而是链接到该表。

SELECT parent.id, MAX(parent.comment) as pcomm,
       pu.username, 

       /* display child comments in order of child USER id */
       GROUP_CONCAT(cu.id ORDER BY cu.id) as children,
       GROUP_CONCAT(child.comment ORDER BY cu.id) as childrenComments,
       GROUP_CONCAT(cu.username ORDER BY cu.id) as childrenUsernames

  /* handle parent submissions, with user identity */
  FROM submissions_comments AS parent
  LEFT JOIN users AS pu ON parent.user_id = u.id   /* get parents' user info */

  /* handle child submissions, with user identity */
  LEFT JOIN submissions_comments AS child  ON  child.parent_id = parent.id
  LEFT JOIN users AS cu ON child.user_id = cu.id   /* get children's user info */

GROUP  BY parent.id, pu.username   /* avoid stupid nonstandard MySQL GROUP BY */
ORDER  BY parent.id, pu.username

正如您所看到的，您可以通过几种调整来获得所需内容。

'字段列表'中的未知列'parent.username'在2个表上执行LEFT JOIN

2 个答案: