php - SQL 子查询返回更多行和更多列

Question

实际中如何解决nested responds to comments ？我有一个包含列的表格:

• comment_id(int，评论主键)
• 作者(varchar，发件人姓名)
• 内容(文本，消息内容)
• book_id(int，另一个表的外键)
• response_to(int，可以是NULL，此表的外键)

我需要 SQL 查询中的这样的数组:

$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(非常低效)

// 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']]);
}

第二个解决方案(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，最多一行。

您可以使用左自连接代替

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