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我确定我看到“多维”和“mysql”一起使用。让我们举这个简单的例子。

假设我想选择 20 个线程，我会让我需要的东西变得简单。我想要 id、author_id、主题和正文。我需要在 where 语句中过滤掉已删除的线程。我想选择这些线程中的所有帖子。我想要 post_id、作者、正文和过滤掉删除等。

我需要知道哪一行属于哪个线程。有没有办法一次性从mysql中选择它？或者我是否需要让它们都具有相同的列数量，放置 IsThread（如果它是一个帖子，则为 false），让每个帖子带有它的 thread# + null 或空字符串主题等。这听起来可能表现更差？（虽然我需要 thread# 当我扔给它一个要收集的线程列表时）

有没有办法获得一个性能良好的简单维护选择语句，它可以让我获得所有这些数据？还是我需要两个陈述？

-edit- 一个评论让我想说这个。我可以List<Tuple<int, int, string, string, List<Tuple<int, int, int, string>> >>从mysql获取吗？是的，这可能会伤害您的大脑，但请记住，我想要通常在两个查询中获得的帖子和线程数据，但在一个

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2 回答 2

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除非我遗漏了什么，否则这听起来像是一个典型的JOIN 查询。

假设您有两个表，如下所示：

CREATE TABLE Threads (
  id INTEGER PRIMARY KEY AUTO_INCREMENT,
  originalAuthorId INTEGER,
  subject VARCHAR(100),
  created DATETIME,
  deleted INTEGER );

CREATE TABLE Posts (
  id INTEGER PRIMARY KEY AUTO_INCREMENT,
  threadId INTEGER,
  authorId INTEGER,
  body TEXT,
  created DATETIME,
  deleted INTEGER,
  FOREIGN KEY (threadId) REFERENCES Threads (id)
    ON DELETE CASCADE
    ON UPDATE CASCADE );

您一次选择所有内容的查询将如下所示：

SELECT
  t.id AS "threadId",
  t.originalAuthorId,
  t.subject AS "threadSubject",
  t.created AS "threadCreated",
  p.id AS "postId",
  p.authorId,
  p.body,
  p.created AS "postCreated"
FROM
  Threads t JOIN Posts p
    ON t.id = p.threadId
WHERE
  t.deleted = 0 AND
  p.deleted = 0;

该查询将生成一个包含八列的表：threadId、originalAuthorId、threadSubject、threadCreated、postId、authorId、body 和 postCreated。

如果这不是您要查找的内容，请发表评论或更新您的原始问题，我会尽力提供帮助。

编辑：

根据下面的评论，您可以通过以下方式优化大量数据的查询。我将使用 PHP 作为示例脚本语言。

// Set $dbh to a PDO object pointing to your database.
$sql = "SELECT id, originalAuthorId, subject, created FROM Threads ".
    "WHERE deleted = 0";
$sth = $dbh->query($sql);
$threads = array( );
while ($row = $sth->fetch(PDO::FETCH_ASSOC))
  $threads[$row['id']] = $row;

$sql = "SELECT id, threadId, authorId, body, created FROM Posts ".
    "WHERE deleted = 0 ORDER BY threadId";
$sth = $dbh->query($sql);
while ($row = $sth->fetch(PDO::FETCH_ASSOC))
  $posts[$row['id']] = $row;

// At this point, you can view your post information, sort it using whatever
// sort mechanism you want, etc. You can get access to the thread information
// in the thread array. For example:

foreach ($posts as $k => $v)
    echo "Post ".$k." created at ".$v['created']." belongs to thread ".
        $v['threadId']." by author id ".
        $threads[$v['threadId']]['originalAuthorId']. " created ".
        $threads[$v['threadId']]['created']."\n";

如果您有大量数据，您可以通过同时向数据库打开两个语句句柄并通过巧妙的排序同时迭代它们来获得真正的幻想。这是一个更复杂的例子：

// Set $dbh to a PDO object pointing to your database.
$sql = "SELECT id, originalAuthorId, subject, created FROM Threads ".
    "WHERE deleted = 0 ORDER BY id";
$sthThreads = $dbh->query($sql);

$sql = "SELECT id, threadId, authorId, body, created FROM Posts ".
    "WHERE deleted = 0 ORDER BY threadId";
$sthPosts = $dbh->query($sql);

$rowThread = null;
while ($rowPost = $sthPosts->fetch(PDO::FETCH_ASSOC)) {
    // You REALLY want a foreign key constraint for the following statement!
    while ($rowThread === null || $rowThread['id'] !== $rowPost['threadId'])
        $rowThread = $sthThreads->fetch(PDO::FETCH_ASSOC);
    echo "Post ".$rowPost['id']." created at ".$rowPost['created'].
        " belongs to thread ".
        $rowPost['threadId']." by author id ".
        $rowThread['originalAuthorId']. " created ".
        $rowThread['created']."\n";
}

（请记住，我只是在袖手旁观，所以提前为任何语法错误或语法道歉。）

于 2012-12-28T06:22:20.997 回答
1

我假设线程与帖子有很多联系。在这种情况下，您可以使用group_concat

例如：

CREATE TABLE services (
id INT UNSIGNED NOT NULL, 
client_id INT UNSIGNED NOT NULL, 
KEY (id));
INSERT INTO services 
VALUES (1,1),(1,2),(3,5),(3,6),(3,7);

SELECT id,client_id FROM services WHERE id = 3;
+----+-----------+
| id | client_id |
+----+-----------+
|  3 |         5 | 
|  3 |         6 | 
|  3 |         7 | 
+----+-----------+

SELECT id,GROUP_CONCAT(client_id) FROM services WHERE id = 3 GROUP BY id;     
+----+-------------------------+
| id | GROUP_CONCAT(client_id) |
+----+-------------------------+
|  3 | 5,6,7                   | 
+----+-------------------------+
于 2012-12-28T06:23:52.037 回答