SQL子查询返回更多行和更多列

Question

在实践中如何解决嵌套响应评论 ？ 我有一个带有列的表：

• comment_id （ int ，评论的主键）
• 作者 （ varchar ，发件人名称）
• 内容 （ 文本 ，消息内容）
• book_id （ int ，另一个表的外键）
• response_to （ int ，可以为NULL ，此表的外键）

我将需要从SQL查询这样的数组：

$result = [
  [ // First comment
     'author' => 'Michael',
     'content' => 'It's very good book!',
     'responds' => [
         ['author' => 'Martin', 'content' => 'I agree.'],
         ['author' => 'Susan', 'content' => 'Hello world!.']
     ]
  ],
  [ // Another comment
     'author' => 'Tomas',
     'content' => 'Hi everyone!',
     'responds' => [
         ['author' => 'Jane', 'content' => 'Hi.']
     ]
  ],
  ...
];

第一个解决方案（效率很低）

// Get all the comments of this book.
$comments = $db->query('SELECT * FROM comments WHERE book_id = ? AND response_to IS NULL', [$bookId]);

// Find all the responds to each comments. 
foreach ($comments as &$comment) {
    $comment['responds'] = $db->query('SELECT * FROM comments WHERE response_to = ?', [$comment['comment_id']]);
}

第二种解决方案（不起作用）

$db->query('SELECT t1.*,
  (SELECT * FROM comments AS t2 WHERE t2.response_to = t1.comment_id) AS responds
  FROM comments AS t1 WHERE t1.book_id = ?', [$bookId]);

Answer 1

错误是因为使用上述方法只能选择一列，最多只能选择一行。

您可以使用左自连接

select *
from comments t1
left join comments t2
on t1.comment_id = t2.response_to
where t1.book_id = ?