[英]SQL subquery returns more rows with more columns
在實踐中如何解決嵌套響應評論 ? 我有一個帶有列的表:
我將需要從SQL查詢這樣的數組:
$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.']
]
],
...
];
// 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']]);
}
$db->query('SELECT t1.*,
(SELECT * FROM comments AS t2 WHERE t2.response_to = t1.comment_id) AS responds
FROM comments AS t1 WHERE t1.book_id = ?', [$bookId]);
錯誤是因為使用上述方法只能選擇一列,最多只能選擇一行。
您可以使用左自連接
select *
from comments t1
left join comments t2
on t1.comment_id = t2.response_to
where t1.book_id = ?
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